I think that the retractable stock on an H&K 91 [or 93] is one of the best accessories you can have. I love mine, and I carry the rifle with it on the gun a LOT, and I have killed a fair amount of game using the retractable stock.
Many times if I am going to be shooting a bunch, or hunting in hot or cold weather, I put a piece of foam pipe insulation on the left rail of the stock, the one that touches my face. I do not tape it on, I just use the split that is already cut in the foam, and slip it over the stock rail. That way I can pull off the foam and retract the stock when I want to...
However, I would also want the regular stock, so just buy your 91 with a regular stock and then buy one of the $200.00 retractable stocks, who cares if it does not match perfectly...
Or if you DO care that it matches perfectly then buy the A3 and then buy a stock set, in green from Cheaper than Dirt for @$20.00...
Then you will have the skinny fore end as well...