Personally I would go for the standard sensation. The sensation has a dual core, sensation xl has a single core. The xl has more internal memory, a whopping 16GB! Mainly to hold a music collection. The standard sensation also has a higher resolution 540*960, the xl has 480*800.
If you want it purely for the music, then the sensation xl over the iphone4. If music isnt that high a priority, I would go for the standard sensation.