My two main saws are a 372 XP and a 346 XP. My back up saw is a 359 that won't work as well as those two in their intended uses but will take the place of either one if they go down.
That's my feeling too, only in orange and black. My 5100 is a good saw in it's own right, but it's not the small saw that my 420 is, or the big saw that my 7300 is. It can do 95% of the wood I cut though, and it's there if one of the others should go down.
For the OP, I don't know what your main saw is, but I'd consider something that uses the same bar mount and chain pitch as your current saw, just for ease of buying spare stuff, the ability to swap out a bar if need be, less different sizes/lengths of chain to keep seperated, etc. Look for a saw a size or two smaller if your current saw is "enough" for what you do, or a bit bigger if you feel outgunned often. A used bigger saw can be had cheaper than a new smaller pro saw if you look around for a bit and aren't in a hurry to have one right now.
Soon your backup saw will need a backup, and then you'll need one with a full wrap handle, then a top handle, then a couple old ones just for show, then...well, ya know, CAD!