As stated there is quite a bit different between the husky 357xp and the 372xp. I own both of these saws. So I can give you a rundown on my thoughts.
The 357xp saw works best with a 18" bar, a 20" bar will work not nearly as effecient as a 372xp with a 20" bar. The 357xp is a good running saw, but should be concidered a smaller performance saw intended for smaller jobs. Should mention on the 357xp its compact size makes it difficult to remove and install chains, the side cover comes off easy, but getting the chain behind the clutch drum on the drive sprocket and on the bar is NOT nearly as easy as on the bigger saws.
On the 372xp the chain drive sprocket is on the outside of the clutch drum, and makes removing and installing the chain a very simple task. The 372xp is a heavier saw, it will run 20" and 24" bars very easy, and is a excellent well designed chainsaw.
seeing you already have a smaller size saw, I would recommend you get the husky 372xp chainsaw for what you want to do. If you end up owning a husky 372xp saw you will be very pleased with it, both power wise and the mechanical design of it.
357xp specs
4.4 HP
weight w/o bar & chain 12.1 lbs
chain pitch 3/8"
My comments: My older husky 262xp will out cut this saw, difficult to install/remove chains, best used on smaller jobs with 18" bar, runs excellent and has very good power for being a smaller size chainsaw.
372xp specs
5.4 hp
weight w/o bar & chain 13.4 lbs
chain pitch 3/8"
My comments: This saw has plenty of power to handle a 20" or 24" bar, even 32" if need be. You need to remember this is a bigger size saw and the rotating mass (crank and connecting rod) is heavier, and this gives plenty of engine torque for those bigger jobs. Its a breeze to change the saw chain, you could do it in complete darkness, its that easy to swap the chain. A very well engineered saw.
Final comments: On the smaller size performance saws such as the 357xp and 346xp, you need to keep in mind the mounts in the saw cant handle the leverage it takes to run bigger bars, and the lighter rotating mass of the crank and rod dont offer enough engine torque to run bigger bars in hardwood.
Good luck what ever you decide..................