Is the rakeback at Party actual rakeback or their VIP-program? It's hard to get 33% unless you play a lot. I think their Palladium level gives something like that, which is easy to clear if you play ~20k hands/month at 200NL or so, but I'm not sure what their bonuses are like at the less-than-Palladium level.
Can't help you with signup bonuses. I assume the $500 deposit bonus at Party is standard at you probably won't find a better one anywhere.
Also, if you're playing Party you probably want to consider switching to the old interface if you're multitabling because the "new" one uses a lot of computer resources. At least it was that way when it came out (and I switched back to the old GUI and never tried it again). For William Hill, you may want to look into getting MiniMaxMod, at least if you're playing a lot and multitabling.