Should I try get Mysterious Heroine X? or Ryougi Shiki?

Leave both and get brynhildr the limited time caster most probably will come in 1st week of February

Do 1 multi and see if you get Heroine X, then save for your preferred banner

Depends on your preference.
Is she a waifu that you want to get? If so, then roll for her. If she isn't, i suggest to skip her. She isn't a bad assassin but she's rather situational and There's nothing a good Archer, Jack or Scathach doesn't already do. Her advantage is that she's "strong" against Sabers with the use of her skill for 3 turns only and her NP makes her stronger against Saberface enemies. And her skills have a long CD, even on lvl 10, which is terrible. That's a rather specific niche that currently (if Jeanne's aren't included), only Artoria Santa Alter fulfils - being a Rider and a Saberface one at that. She is a middle of the pack Assassin in my opinion so i'm skipping her because i overspend on Moedred banner and as a F2P player myself, don't need to roll on her banner.

Not even CEs from the gacha are that good on their own, except for boosting drops for the event so you aren't missing out on anything if you don't roll in the MHX gacha. She's a limited servant though with at least one more rate up in the history of JP banners so she's rather rare but Brynhildr is rarer.

TL;DR

MHX is a niche 5*star. She is completely replaceable with a good Archer and overshadowed by Jack, given her niche. Shiki (both a free 4* and a 5*) are much better servants so my suggestion is to save for her if that's the plan.

i suggest you to save up until your favorite banner came.