I have had 2 346 XP's, 18", owned by the Co., but pretty much used exclusively by me. The 1st was stolen, so I immediately got another 1. It tops out at 14,500 RPM. It cuts much faster and smoother with the narrow kerf chain, Oregon 95VP.
I have used 357 XP's, 20", equipped with .325 chisel chain for ground cutting only and was also impressed. It tops out at 14,000 RPM.
The 346 is 1.1 pound lighter, but he 357 has more horse. I would think it comes down to your personal preference. Do you want more horsepower with the 1.1 lb. added weight and the additional $?
I was perfectly satisfied with the 346, it was my crane removal saw of choice for up to 20-21" cuts.