For me, it was an easy decision. Mountain of Faith was my first Touhou, has always remained my favourite, and is simply the one I know my way around the best. I've achieved Hard 1ccs elsewhere (IN, SSS, close to one in HSiFS) but MoF is the only game where Hard 1cc is comfortable; I can start a credit knowing I'm going to achieve it, and it doesn't matter if I make silly slips here and there.
For my Lunatic 1cc, I'm using ReimuA (homing) because then I don't have to spend time learning stage portions. The spellcards will still be tricky, but the spellcards took the most learning effort when I was going for my first Hard 1cc, so I know them pretty well by now.
I was also stuck for a very long time feeling that Lunatic might be forever out of my reach. What got me to the point of seriously going for it was a combination of things: (1) I got some great feedback on my runs from Touhou Replay Showcase, especially Normal scoring or Normal challenge runs (e.g. no focus or no vertical or no bomb), and Maribel outright said that in his opinion I was ready for Lunatic; (2) improving my general skill by going for high-fps achievements, such as Normal and Extra 90fps; (3) the release of HSiFS. When HSiFS came out, I struggled enormously at first, just because it was so tempting to do risky things to get a good reward from releases; but I kept at it until I could actually do those things, and I soon found that my overall play had gone up a few notches.