The S545 might be the better choice, overall.
The larger capacity is the clear winning feature. SensMe is just a round-about alternate to playlists, which you can easily make on your computer. Aside from that, the two players aren't going to really differentiate themselves aside from memory.
If you really wanted to save some cash, you could go for a Clip+ or Zip and grab a big microSD card for it. That would get you the same amount of storage (or more, depending upon the price of the card) for less money. (cue the RockBox comments...)