Reliability-wise, it's been a wash for me. I've had one alu shutter that was warped on a junker FM2n that I got to resurrect another one. Still fired, but definite light leak. And, I've had one titanium shutter on a junker early FM2 where one of the curtain blades was off its track. Still fired just fine as well. So far, I've owned 19 of these bodies over the past 19 years - 18 of them starting in 2005 and continuing on, thanks to the market falling out on film gear at that time.
I have noticed, on a shutter tester, that the titanium shutter was slightly more accurate. Not enough to make much of a difference in exposure, though. So, whichever shutter you have will likely be reliable.
-J