Well it seems that the offerings and prices at both the Senses Spas (at Grand Floridian and at Saratoga Springs) seem comparable. I think the only other on-site spa would be the Mandara at the Dolphin (possible some of the DtD area resorts have spas too). I guess what constitutes "romantic" would be in the eyes of the beholder and might also include location. According to WDW's website about their spas (hardly the most unbiased source), Saratoga's Senses is reminiscent of "New York's legendary mineral springs" while GF's Senses maintains something akin to the Victorian theme of the resort. Your Girlfriend might appreciate the afternoon tea served at GF; then again she might prefer a walk or boat ride to DtD after her spa for a shopping excursion or the balloon ride. Or going to the more Asian themed Mandara affords you time in the Epcot resort area, boat ride to Epcot or Studios, walk along the Boardwalk, etc.