I believe the 2588s got a heavier front axle, but I don't know which model year started that. The 7088 would also have a heavier rear axle. As for the sieves, they are the same all the way from the long sieve 1688 on up to the 7088. The major difference is HP.
JD is in a similar situation between the 9670 and 9770. They're basically the same machine, with the difference being HP. Sieve area is the same between them. If you can use the HP, then go with the 9770, but otherwise you could save some $ and go with the 9670.
We run 8 row corn heads and 25' platforms on a 2388 and 2588. We have found that we need all the HP we can get in green stem soybeans. In good dry beans or in corn, HP is rarely an issue.
-Lance