I've done three trips (April 2011, oct 2012, oct 2014). This last one was the only one where I took out at Diamond, and I wouldn't do it again. It takes longer to get home, shitty dirt road, expensive Hualapai fees, and you miss some excellent (and some of my favorite!) parts of the canyon. Remember: if you have 18 days on your permit, you have 18 days to get to Diamond; the rest is bonus! Take your time to at least Separation, then create a party barge the rest of the way. The camping is pretty dismal for the last couple nights if you don't do the full moon float, and yes the helicopters can be bad, but I think it's totally worth it. Plan to sleep on your boat and be sure to bring enough beer (buy what your think you need, then double it) and you'll be glad you went the whole way. The chances of Lake Mead ever coming up high enough to back the water up past Pearce again are pretty low; you definitely don't need a motor.
My experience is that there's more wind on spring trips than fall (goes for any desert river), but I'd still bring a Bimini, especially if you decide to go all the way to Pearce as it can get hot down there. I'd also bring a drysuit.... I like to go prepared for anything (it can snow in April) and I've always been glad to have it.