Are you talking about diving with neoprene exposure, or something non-compressible? With a compressible suit, I would definately lean towards the 45#. With a non compressible suit, the 30# should be more than adequate.
A steel 130 holds about 10# of gas, an AL 40 if you choose to dive a redundant bailout with singles will hold about 3# of gas. That's 13# of lift needed... [well, 16# since you need 3# extra lbs of lift on the side with the AL40]
If you are diving wet, or with a neoprene drysuit, then you have to take into account the compressability of the suit... in which case 45# might be a little more reasonable.
Too much space in your wing makes for annoying trim issues should your gas shift in the wing. I dove a 55# wing with singles for a bit, and it lasted maybe two dives post cavern class [after removing the gosset bungee]. The shifting of the gas was just a pain to keep in control.