Still I would go for a GT4 drive because nowadays the price difference between the GT4 or GT2 SSD's is not that big anymore (~10 Euro/Dollar) and still there is a (very very very) little change that Dell will update the PCIe interface to GT4.
P.S. to make your choice a little bit easier, compare: - Total Bytes Written (TBW) - Random 4K, QD32 (IOPS) - Warranty - Encryption!
P.P.S. If you find a good SSD GT2 offer, buy that one!