I haven't chimed in here in a while, so I thought I would. I still haven't settled on what SLR I'm going to get since selling the FM3a. I really love my M6 and have so much invested in it, I can't justify the purchase of another expensive camera that will sit around 90% of the time. Right now I have two cheapies, and OM-10 that was my fathers which I will never get rid of, and a crappy Chinon CE-4 that works OK but could use a bit of cleaning. I got it with two primes, a zoom, and a flash for $25, so even with its dubious operating condition, it was a good deal. Despite my love for the OM-4 and the fact that I have another OM body as a backup/second type of film camera, I'm leaning towards a Pentax MX or some other K-mount body, maybe another cheap one like the Chinon, then just investing in Pentax lenses.
Out of all the old SLR systems, the price to performance ratio of used Pentax lenses seems to be in the buyer's favor. I'm really interested in the old SMC Takumars as well as the SMC-M lenses, and they are so cheap, it seems too good to be true. I'd love an LX, but they are just too expensive despite how nice they seem. I could probably find a K1000 for next to nothing locally, but they are a bit too big for me. So, if I get a proper Pentax body, it will probably be an MX, otherwise it will be Chinon, Vivitar, Ricoh, Cosina, or whatever is cheapest and "light tight".
Also, I go back on one of my earlier statements that the FM3a is "too big". It's not too big compared to many other SLRs, as others have pointed out. However, it is also not the smallest, and there are bodies that are closer to the M6 form factor and feature things that the FM3a doesn't, such as a bigger and brighter viewfinder. Again, it's all personal preference.