Summer or Next Jalter Banner

Speaking from experience, Jalter and any solo rate up SSR can take a ton of quarts to pull. Easily 300+ unless you're lucky. You do have better chances pulling an SR but because Martha shared it with Caster Marie it lowers that chance. My advice, don't worry about an Extra right now. Either pull for Jalter or Merlin. Save at least 300 quarts for Merlin, spend the rest on Jalter. If we're being practical though, skip Jalter and roll for Merlin. You've already got Ozy who is a fantastic ST servant

If all you want from summer is Martha, then skip it. You have a ~slightly better chance of getting Martha than Jalter, but it's small enough not to consider, and Jalter is honestly much better. If you want to be that much more certain of pulling Merlin, then I agree with the other reply, save exclusively for him. That said, consider that there is another Merlin banner next year, while Jalter will only appear in paid guaranteed SSR gachas until 2020.

Rolling for support servants are at an odd stage at the moment. It's quite apparent that you're inevitably going to have friends that have rolled both Jalter OR Merlin meaning that you can drag them into battle at anytime with the merit of gaining a lot of FP in the long run. If you must have these servants in your team then consider this:

1. Merlin works well in a buster/crit-oriented team. If you have a main damage dealer (in this case, Ozy) and a star generator (Jack) you can plow through the most difficult challenges currently in game and in future updates. He has as much, if not, more utility than a Waver.

2. As a Jalter owner, it saddens me that she becomes a little less viable (still extremely strong) in future updates where bosses have more than 1 HP bar. Because of this, it becomes harder for her to burst them consistently and prevents her from 1 shotting enemies with that type of HP pool. She might need some aid from a support servant, but she can blow up almost everything with an NP you can charge in a few chains.

3. Martha's hard to kill because of her skillset and class meaning that she can be a good addition to a stall team (less applicable in an arts stall team). Despite all of this, she lacks any form of team support skills or good offensive buffs.

All in All
Merlin: Broken; slap him on any comp.
Jalter: That's a lot of damage.
Martha: Doesn't die and does a decent amount of damage.

If I were you: Martha < Jalter < Merlin