All ATi Radeon 9000/9500/9700 boards are clocked identically regardless of manufacturer and OME/Retail status, there is absolutely no difference in clockspeed.
Thankfully ATi has finally tightened up their third party clockspeed requirements somewhat so one no longer needs to worry about clockspeed variations as you did with the 7500/8500.
Whether this will change in the future is open to debate, but for now it doesnt matter whether you purchase Retail or OEM. If it's an R9700Pro it's coming clocked at 325/310. If it's a regular R9700 then it'll be clocked at 275/275.
Additionally, all ATi Radeon 9700 Pro boards are manufactured by one company, the only company that presently plans to manufacture their own models is Tyan and they've yet to release their R9700. Regardless of whom you purchase from third party or ATi, you'll get a board manufactured by Saphire.
If you want an R9700board manufactured directly through ATi's own fabrication facilities then your stuck with the AIW R9700.
So personally... I'd go with OEM.
I don't need the software, annd I'd only keep the board for slightly over a year in all likelyhood so the longer warranty would mean little to me.
If you want a 3yr warranty, or if you want the bundled software then buy Retail. Otherwise get OEM.