The only reason I'd opt for a 135 over a 175 is if I had NO WAY to access to a 20 amp 230V outlet. (And the 135 needs a 20 amp 120v outlet.)
By my research the price difference between a 135 and a 175 is not much of an issue (roughly + $140 for the 175.) considering the increased capability.
The 175 will have a slightly better duty cycle than the smaller machine (175 is 30% at 130 amps where the 135 is 20% at 90 amps) This initially may not be an issue... but that's your call.
The 210, 251 & 350 are in a whole other class of machine. If you have the 230v power input... don't require "portability" and can stand the higher price tag, any of these three 'industrial-class' machines will make your day. The 210 starts at aprox $1300, and the 251 is aprox $1800.
No doubts that a skilled welder can do some great work with either a 135 or a 175, but the larger rigs will increase productivity and capability.