Of those three only Dreamspinner is an epub. I don't know about them specifically but you should make sure you are aware of the typical sales volumes of your options.
From Brenda Hiatt
Kensington/Zebra
Average advance (first book): $3600 Median: $3000
Average advance (subsequent books): $7100 Median: $3700
Advance range: $1750 - $60,000
Standard royalty percentage: 6 - 8.5%
Average earn-out: $6200 Median: $3800 Range: $2500 - $17,800
Kensington
Average advance: $2850 Median: $1550
Advance range: $750 - $9000
Standard royalty percentage: 2 - 2.67%
Average earn-out: n/a
From EREC
Average erotic romance ebook sales as of March 30th, 2009
First month: 239 copies
First year: 406 copies
Total to date for books out one year or less: 409 copies
Total to date for books available for more than one year: 701 copies