The Gti is an older model - I have one with almost 200,000 rounds through it - replaced the springs and firing pins at about 90,000 - the only maintenance repairs done to the gun. I also have an Ultra XS(similar to the sporting) in 20 gauge with 30 inch barrels -it has about 20,000 through it with no issues.
I am surprised to see a sporting without porting. Browning ports EVERY target gun, but not their field versions. You might be looking at a field version - if so, it will tend to be lighter than the target version. Personally, I prefer the heavier target versions for clays as it helps with recoil mitigation.
The basic fit will be the same, although, IIRC, the XS sporting comes with a schnabel forearm while the Gti came with a beavertail - a semantic difference to some, a major point to others