Our experience as a flight school comes from operating at a busy international airport (Bilbao), private aerodrome (Los Oteros) and our current base at an airport with very little traffic (Burgos). From ATC to non-controlled to AFIS service… in our humble opinion… there is not a straight answer. It depends on other factors…
I would say mainly existing traffic. I would personally not recommend training at a super busy airport because it will cause delays (and frustration). But commercial and other GA traffic included!
For us, operating from a busy commercial airport was a nightmare both for students and the school…
It is nice to be able to complete all training from your home airport which will save you time and money for the more advanced phases… but it is also true that you will surely visit many different airports during your cross-country flights…
Learning communication skills is also mandatory so you don’t want to be completely away…
Traffic congestion in the area should also be considered…
Bottom line… it depends. There is not an answer that fits all cases.
My advise would be to prioritize non-congested airports and air space than ATC but I am pretty sure many people will have different views about this matter
cheers