I just wrote a really detailed post and it got erased....as it is getting late I'll summarize.
I am an 18 index and just bought the 52s. 300s were next on my list (and almost tied). I preferred smaller head and overall feel of the 52s. Forgiveness on 52s is a TAD bit less, but nothing significant in my opinion. I almost went a mixed set 4-6 (300) and 7-W (52) but changed my mind when I tested the 5i side by side and preferred the 52. Couldn't hit 4i all that well, so going with 3/4 hybrids. Also hit the MP Fli-Hi 4i. I can't understate how good this club is. Almost prefer to a hybrid. I was amazed at the loft and distance I was getting.
Whatever you do, see if you can try a fully fitted club (shaft and lie). Shaft makes a HUGE difference in a club - don't underestimate that. The nice thing about Mizunos is the fitting club that is simply amazing. It makes things a lot easier. Take 4 good swings and it recommends 3 shafts. I went through process of elimination and was fortunate to be able to try a fully fitted 6i 52 vs. 300. Next on my list was the AP1 - although the flight path is fairly high in stock shaft. I didn't bother to demo other shafts because I really liked the smaller head of the 52s. Next on list would probably be the MX200 followed by the G15 (although I never hit the G15 on the range).
Back to the question - can't go wrong with the 52s but demo all you can and start weeding stuff out. It is a great time to be buying clubs - lots of quality ones out there right now!
Good Luck!