I have used the 5100 in the past and currently own 2x 5105's and 2x 346xp's and a ported 550xp. Have lots of time on stock 550xp's also
Despite having so many top 50cc pro Husky saws I still use the 5105's quite a lot. Depends what i am doing. The better inside and outside spikes on the Dolmar mean i can more easily do much bigger bucking or felling cuts compared to the crappy tiny spikes on the 550xp and 346xp. For limbing of standing trees or stripping a felled trunk then its always the 346xp.
My ported 550xp is an amazing saw. So much power in the same light and nimble chassis. I seem to notice the balance and handling of a saw much more than say a tiny 1/4 lb weight difference.
Stock for stock i would probably go for the Dolmar over the 550xp, simply because i have had them longer and know i can trust the reliability. If i had better bucking spikes on the 550xp and the AT system was a little less temperamental then i might change my mind.
I personally don't think you can go wrong with either saw.