The differences between Chevy and GMC are slightly more than just trim packages but you have to pay attention to the year(brought it up just in case you found a decent older vehicle)!
Mechanically and other than a few years here and there where there were different motor, 4x4 and suspension options , they are identical.
Chevrolet advertises their products as entry level and offer more models, there are less options to choose from and the prices reflect this. GMC has less vehicle models to choose from but offer MORE options on those vehicles and you also pay for it. More Chevys are sold than GMCs. If you want a good foundation for smaller price go Chevy. If you want more choice in the bells and whistles you'll find it with GMC.
GMC had a 6.2 in their motor options and Chevy didn't(max was the 6.0) but I forgot which years and models this occurred with and I believe this was prior to the mid 2000s so you might run into it. Also there have been various years where 4x4 offerings were available to some models but not to others. Recently GMC has been offering different fenders and bed side panels on their trucks and camper manufacturers had to adjust because of the different bed sizes so now not all campers are interchangeable.
You also have to pay attention to the years of the product you buy when it comes to classic metal. Way back when Buick, Pontiac, Oldsmopile, GMC and Caddy had many interchangeable parts but Chevy did NOT until the 70s maybe(I think it was the early 70s IIRM).
Oh almost forgot, I think it started with this model year but the dash trim GMC offers is aluminum, an upgrade to the plastic on Chevy models. The Escalade now offers Magnetic Ride control and if it pans out economically I'm assuming well see a basic version of it appear for GMCs in the future. Just a guess of course.
As far as the differences between the Savana and the Express; it's claimed the Savana rides better(not sure how) and there were a few years where the Express had a different wheelbase than the Savana. The Savana has more standard options. That's pretty much it.
Yes the GMC is considered more deluxe and the parts are not only interchangeable, they are the same. Both model vans are built in the Wentzville Assembly plant, MO. I've pretty much always preferred the look of the GMC over the Chevy but when it comes to finding one I'm going with condition, mileage and price.
https://en.wikipedia.org/wiki/Chevrolet_Express
A serious good luck on finding your dream ride. Vans don't grow moss for long even a lot of junkers I've seen and sellers know it.