Gramps or Jalter?

It depends on your playstyle honestly.
Jalter is a damage dealer with busted self buffs through Self Modification and Ephemeral Dream. She works amazingly with Merlin, who boosts her crit damage on buster cards to astronomically high numbers
She requires slightly more effort in teambuilding, since she can't really make a lot of crit stars for herself. Her relative lack of survival options means that one crit can break her.
If you prep her well enough though, she can mow down any boss with a brave chain.( Until break bars come in lol)
Hassan is an assassin whose main focus is being assassin cu alter with stupid amounts of debuff resistance.
His BBBAQ deck and high attack for an assassin means that he can consistently deal decent damage even without buffs, or having a team built around him.
He does not slack on his arts and quick card however. He has amazing NP gain and decent star generation, meaning his arts and quick cards are not useless
He has amazing survivability due to Protection of the Faith sporting a decent heal the best BatCon in the game. His debuff resistance on Protection also means that it is unlikely he will get debuffed, thus making him decently consistent in most situations, at least when Protection is on.
His flaws? None to talk about other than his assassin class, which admittedly is a pretty terrible class.
In conclusion, Jalter is a glass cannon that deals impressive damage, though she trades some survivability for that while Hassan is a jack of all trades assassin, though he is held back by his class.
Personally? Hassan. I like consistency.
However, considering you have servants such as Okita and Dantes, you might be able to solve Jalter's crit star problem

Both have their pros and cons. Be ready for the lengthy wall of text that you will now face.
Jalter:
Pros:
- Amazing Damage
- Charisma skill
- Works well with Merlin
- HIGH NP Gain due to being an Avenger and having high NP Generation on her attacks
- Takes neutral damage except for a few classes (Helps with her low health somewhat)
- Tsundere
Cons:
- Pops like a balloon even with Merlin this is due to her low health even if she's an avenger
- 3rd skill has both your damage and survivability
- Is an avenger so she does neutral damage except for a few classes (But I guess it's fine since she has high base damage)
- NEEDS support to survive
- Tsundere
Conclusion:
Is a really good servant overall you just have babysit her for her to bring out damage.

First Hassan:
Pros:
- High Survivability (This is due to him having the HIGHEST level of battle continuation)
- Almost as durable as a Ruler (Almost but good try though... Protection of the Faith provides a high defence up)
- High Sustain (Again Protection of the Faith reason why he has so many solo videos)
- High health (Trades it for his attack stat)
- COOOOOOOOOOOOOOOOOOLLLLLLLLLL (Get that reference? But seriously guys so badass.)
Cons:
-Piss poor damage (Don't you say things like but he has 3 buster cards, Protection of the Faith has an ATK up or the just use Merlin LOL. Dude he's an Assasin assasin modifier effectively to the 10,000 or even less)
- Low NP Gain ( Due to having 3 buster cards and yes he does have HIGH NP gain but its tied to his 1 arts card to solve this you need to card count to prepare and for stars or just have 2030 on Merlin in order to charge his NP Bar a good bit)
- Needs Support kinda (He can kinda solo but future events in JP say otherwise lol. Kidding aside he needs support to counteract that assasin modifier.)
- Not a waifu (I'm just joking and I'm not telling you not to roll for him since he's not a waifu he's so badass in Babylonia that you might want to consider rolling for him.)
Conclusion:
A good servant to have because his survivability is off the charts but trades his damage for it. Needs support to deal with it somewhat

Who to roll for:
I'd go for Jalter not because I'm a fan but her ability dish out damage from spamming her NP every 2 or 3 turns is quite good. But the main problem for both of them is the introduction of breakbars so the burst damage on their NP's and Merlin-enhanced buster cards / Buster NP's will have their potential cut as opposed to Arts and Quick teams using back to back NP's. And that's the reason why the both of them are so below the tierlist on JP right now and figthing against gimmicks during events scew the both of them (Jalter and First Hassan are still really good in the story just really bad at event challenge quests that have enemies with a ton of breakbars and even if you have Merlin you can't out-sustain them or deal with their gimmicks). But Jalter as I stated earlier can use her NP multiple times which makes her really good at story and decent at challenge quests. I may sound like I'm shitting on First Hassan and being biased but you said you will be rolling for Merlin so having high damage will make Jalter benefit more from Hero Creation.Or if you really want roll for both since you seem to like both of them on one hand you could get a great Avenger (Best avenger is Dantes lol since Skadi made him op. So high in the tierlist now be happy about your Dantes. Nah I'm just kidding Jalter is better individually as a servant compared to Dantes but with Support Dantes is probably above her.) and on the other hand you could get the second best or best (opinions may vary but Jack just brings so much more to the team and Fate/Strange Fake Jack was so cool and lovable he can even turn to loli Jack the ripper on some parts of the story just for the lols / FBI Jokes and trolling Flat *Dude Flat was like go back to your other form the police might come and covering her with blankets and shit since loli Jack the ripper is still on her skimpy swimsuit* XD.) Assasin in the game (His only competition is Jack and Jack's still really good with her RAW stats and HIGH DAMAGE NP SPAM but she has really low health and pops like a balloon as opposed to First Hassan Survivability and Sustain.) And no I don't believe in Appmedia's tierlist but their ranking was actually accurate since they both have a hard time on challenge quests but they rated them both unfairly since the two of them are still really good at the story (I mean look at Musashi so fcking low but I guess she needs support to dish out damage and her skills ALL LAST 1 TURN but I'll just roll for Musashi during Shimousa banner since Ishtar is priority and her banner has good CE's). And lastly goodluck on your rolls on the gacha and spend wisely fellow master of Chaldea.