Grab that RS opportunity asap. Keep the cab, both for $$ reasons and also to avoid GT3 redundancy.
If you sell the cabrio, you may lose $20K+ off original MSRP. If you sell the GT3 now you will take very little hit as far as depreciation.
Do you get a tax offset for doing a dealer trade-in? If so, see if your dealer will allow you to find a private buyer for your GT3 (I would guess finding a GT3 buyer will take only a matter of days-- at least in the here and now, you could probably expect 95% or more of original MSRP), sounds like your dealer is a good sort, see if they will run the GT3 sale 'through the dealer' which would technically make it a trade-in. (perfectly legal, just a paperwork shuffle with dealer owning the car momentarily.....I did this with my first 997-- my transaction was in Ohio-- Ohio BMV actually gives one year from trade-in date to take delivery of the new car and only pay sales tax on the $ overage, saved me around $6500 in taxes to pass the sale through the dealer).
From talking to several people who have spent decent seat time in both GT3 and GT3RS , the regular GT3 is a dream-- the RS is like a GT3 with permanent shot of adrenaline.
Sounds like quite a long strange trip from your Bimmer M3!
What color RS you thinking of? if you want to stick with carrara white, the white with black graphics would be sweet...