A few comments:
1. Diamond has the Pro/Am, the Professional, and the Paragon, in the commercial space. They all play exactly the same, as the under-the-hood stuff is the same on all 3. 99% of the Diamonds you will see are the Pro/Am. Make sure you see pictures of both the Pro/Am and the Professional, because aesthetically, they are completely different.
2. The Professional can be ordered with rounded corners, or 45 deg corners. Both give a different look.
3. The Professional can be ordered with 2 different leg types. Both give a different look.
4. All of the tables can be ordered with or without the big Diamond logo engraved into the end.
5. The main difference in play between a GC and a Diamond, is how the balls react off of the rails, and how the pockets accept the ball. You will have to find a Diamond for yourself to see if you like these two characteristics more/less than a GC.
6. The shelf on a Diamond is much deeper than a GC. So, a 4.5" pocket on a Diamond plays much tougher than a 4.5" pocket on a GC.
7. Diamond tables can all be ordered in 3 different pocket sizes. "Pro-Cut" is their standard pocket at 4.5". This is what all of the tournaments are played on. "League Cut" is bigger at about 5.0" and is a special order. They also have a tighter cut, I forget the exact name, usually ordered by one pocket players. Its about 4 3/8" if memory serves me.
8. GC tables from the factory measure 5.0" at the corner pockets. If you were in a "player's" room in the 80s, the owner may have had shims added to some or all of the tables. A "double shim" GC, common for 9 ball gambling, was about 4.75". A "triple shim" GC, common for 1 pocket gambling, was about 4.5".
Good luck and have fun.
As far as a your reasonable price claim, I don't know if I entirely agree with that - $5,500 for a 7-footer and likely at least $1,000 more for each foot bigger than that. Many of us may not consider that reasonable, even for a commercial table.
I don't really agree with that, demand for the Diamond tables far outweighs Diamonds ability to produce them, therefore someone must not feel they're to high priced. The only time you hear anyone complaining about the price of something in this industry...is from Americans who always seem to feel they should be able to buy cheap and sell high if anyone wants their business. Same thing happens in the cloth industry. You're not going to get a Rolls Royce for the price of a Volkswagen.