My thoughts are this.
I own a 5100-S and it is a great little saw. I know some people talk about balance but to be honest with the smaller saws I don't believe it is as big of an issue as it is with larger saws and top handles. I've thrown my 5100-S around for up to 10 hours clearing Wattles in Tasmanian creek lines and not once did I stop and think I needed a better balanced saw.
If buying again though I would narrow it down to the 346 or MS261. Why you may ask?
Simple, the standard filtration on the 5100-S is garbage (and I believe similar on the 5105) in our hardwoods and has to be oiled to keep the crud out. I know the 346 will handle it and the filter on the 261 "looks" like it should handle it.
I have thrown a 260 around without actually cutting and it felt a bit lighter but balance is highly subjective and depends a lot on many factors...
Personal choice
Brand bias
What saws you have actually used
Intended use and arm strength
and last but not least, how anal you are...
When you see somebody mentioning which saw is better than that saw always make sure they've spent some time on all of them before jumping to conclusions. There are many members here that will try to convince you to buy one saw or another when they have no real experience on any of them.
I would lean towards the 261 only because I don't like outboard clutches when there is an alternative, similar saw that has got an inboard, but that's just me. Others will disagree
If clutches don't bother you I'd go the 346 as it has an awesome reputation and I don't think the 261 has been proven yet but I'm sure it will be over time.