Thanks for asking.
A top end from either company will cost the same.
I guess it depends on the type of riding or racing your going to use the engine for on where to get it from, if you want a CT or an ESR ported one.
CT, those p/v valves work better at bottom end than the ESR valves on trail, GNCC, or MX. If your play drag duning or later want to drag port the cylinder, then the ESR is fine.
Far as porting, the CT will be more fine tuned than the ESR. I've had my hands on many of the ESR ones & I'm sorry but thats just how it is.
Now, if you were one of their sponcered riders I'm sure it would be different & that would be from either company, ESR or CT.
What size is up to you, what fits you best for what your going to use it for.
A 310 is just a matter of bolting it on. The 330 you would need to have the cases cut to bolt it on.
ESR has a bolt on 330 with short skirts so no case cutting is needed, but I dont know if they have it in the p/v version.
Far as the 330 being tiring, porting make alot of difference either size of cylinder but suppost to be bigger the piston is more power it should produce, not always true in some cases though.
Just for giggles for me, far as the tiring part I have a cure for that

get a lawn chair & let sombody else ride it, thats just for me though. I love to eat to good, if you know what I mean.
I can get you a ported top end kit from either place, or I could port one for you myself, price would be the same as the other places but the porting would be different from me.
Matt, far as I know still has a used 310PV for sale with my porting, might could look at that.
Neil