Having had 2 MS3s and 2 MS6s I can say that in reliability MS6 and MS3 are very comparable. Parts availability is also about the same, so I wouldn't worry about that too much. It really boils down to what you are planning on doing with it: if you're not planning on going crazy with it then they are about the same cost-wise. If you have plans to do a build with significant power increase, then there's more to spend on the MS6 but it also has more fun potential.