I wouldn't change from Falco to Sheik because you think she's better or because you are doing better against your friends in the short term. Characters have different learning curves. Falco, in my certainly biased opinion, can change pretty radically from one skill level to the next. At the lowest levels, he's awful because you fall off stage and can't recover. A little better and he's good because he has strong attacks like fsmash/dsmash, bair/dair, and lasers that you can spam. For the average player, the other characters seem to catch up a little as they are able to start stringing together simple 2-4 hit combos and get the occasional gimp. Above average he gets even worse as the combos hit harder, his speed starts to become a bigger problem, and lasers stop working as an auto-approach. At the highest level, he seems to compensate for those changes by simply playing smarter and getting hit/grabbed less and optimizing punishes. Ultimately, he seems to be right around all the other top tiers in terms of potential. I think the "Victory Road" segment just before that level happens to be rougher on him, but again, that's all imo.
Really, if you are just trying to decide between Falco and Sheik, just play both. If I had to guess, I'd say your new interest in Sheik is likely a phase (one almost everyone goes through with a secondary, myself included), but as you get better you'd probably regret ever dropping Falco completely. So by all means shift your focus onto Sheik for a while, but keep your Falco trained up so if you change your mind you don't feel like you screwed yourself. Especially since your main complaint seems to be performing tech skill, you probably just need to practice more. I'd hate to see you switch to Sheik for a quick, short-term improvement only to have the novelty wear off after a month of not playing Falco.
