You can't use power as a that sort of choice for a decision. Doesn't work that way, plus the whole thing with SB and PB, ie PB 250W gives you several dB advantage.
I'd try and get the best single sub as you can now (without overspending insane amount, so a second is still acceptable to buy) then get another one later - but I wouldn't leave it too long if they discontinue that model.
OP needs to go listen to few brands, choice of brands is overwhealming I could name 20+ off the top of my head. For Hifi it's even more, go to Hifi /AV show and most of them never even heard of. Then for subs, you might find specialist sub makers better rather than the same brand as the speaker. ie Q Acoustics subs, pretty bad.
Also for the OP he should decide if wants to be a hobby, ie starting off with a cheaper system, enjoying it, upgrading L/R, replacing other bits, buying movies, upgrade sub, remote control etc. That's part of the hobby.
Ie you could start off with this, and a cheaper AVR, everything there for about £1000
Wharfedale Diamond 9.1 5.1 Home Cinema Speaker Package, Black at AV.com
www.av.com
Keep it for a year or two, enjoy it, go and learn more about this, go to Hifi/AV stores, shows etc, when or if you decide to upgrade move the speakers to a bedroom, workshop, PC system, kids room etc. Probably safer that way, imagine you spend £10000 on a Klispch speaker system and wondering why your ears are bleeding after a movie....and then find out the Klipsch sound signature. Also learn how to connect, setup and wire up yourself.