You would be making a wise decision going for P+ rather than Pbpro, even current Pbpro users acknowlage that. Just look at the GLCD, data handling and Variable support to compare.
I'm not sure what you mean about Upgrades, Crownhill is constantly improving the functionality of Proton unlike Melabs, so you can say they upgrade there software constantly.
Crownhill also have a free Update policy, bug fixes general improvments are posted for free. However I would not use the word Upgrade as that generaly indicates a very large step in it's development and the last one was not free, they like every other software producer in this world has to cover development costs. You never see microsoft offering Word updates for free.
Saying all that though I am sure that the current purchase price includes any near future Upgrades, so I can comfortably say you can purchase now with no worries.
Proton has built in a 98% of Pbpro comand compatability, and those commands not covered 100% only require a little thought to swap the syntax around to make them work.
Proton can be as easy or as complex as you want it. Dispite being very simular in it's syntax with Pbpro I would say it's a lot easier.
Simple example
To Clear the screen and print on line 2 "hello"
Pbro
LCDOut $FE,$86,$FE,$0C,"Hello"
Proton+
Cls
Print at 2,1,"Hello"
A = 100*656 is not possible in Pbpro as the answer will be to big for it to handle.
Proton can easly handle say
A = 100 * 2684354