Medb or Helena?

Just pick who you love the most. Utility and such, you can think about it later after you actually get them.
Roll for waifu and not gameplay, anyone can work if you put your efforts and head to make them work.

Well, if either is fine I would just point out that as a 4* Helena is much easier to get than Medb, and you would probably have more quartz left over for a shot at Jeanne Alter.
Of course, you could theoretically still spend everything and not get Helena, or spend one ticket and get Medb because that's what gacha is like, but mathematically between Medb and Helena (and also Jeanne Alter) if you count them equal in your desire, then the best choice is to go for Helena. She is the least likely to spurn you.
Of course, I suppose there's some additional math to be considered in that Helena is still sharing a rate up with Rama, but I'm still pretty sure she's statistically more likely to appear than the 5 stars.

A few things to consider:

First off, generally speaking, you have much better chances with Helena since she's a SR. But for that same reason, if you don't get her now, there's still a pretty good chance you'll get her not even trying to roll for her later down the line. Medb can theoretically also spook you later, since the moment you finish the story she's part of the regular gacha just like any normal SSR, but her rarity means your chances are lower. Also, Helena being both an SR and part of the normal gacha as is means that even if you roll for Medb you have a decent chance of getting her on the way. I really wanted them both too, but that's why I rolled in Medb's banner, so it's what I personally would suggest.

Secondly, about Jeanne Alter-- she's a limited servant and her banner should be pretty soon, around early April, but as far as limited servants go she does have some rate-ups later on. Not a lot, but a decent amount in the future. From what I remember, there's a lot of time between those rate ups, though, so it also depends on whether you want her now or are willing to wait. (The time between them means you also have a lot of windows to save up more quartz, so the wait could potentially be very frustrating, but it won't be useless.)

If you want her now, more so than Medb or Helena, I'd suggest holding off on these two and saving up your quartz for her, and waiting for Medb and Helena's future rate-ups (which are a fair bit more frequent). But if Jalter is a lower priority, and you're willing to wait a few months, you can pass on the banner, roll for Medb/Helena, and save up your quartz. (Of course, if you're successful enough with these two and have some leftover quartz you can also try your luck with them on the upcoming nearest banner, but that really depends on your results, as well as how much quartz you have now.)

Anyway, apologies for the overlong reply! But basically it's a matter of priorities. Either way, for Helena and Medb, my suggestion is going for Medb and hoping Helena either comes along the way or spooks you later. For Jalter, it's a matter of who's a higher priority for you between the three, plus whether you specifically want to get her as soon as possible or want her but are fine with waiting for a later banner.