If you really will never go in the summer, an Aug UY is ideal. However, if you have a lead on an April one and it meets your price, then I agree w Carol, an April UY can be fine too. Look at how they would work differently for you:
January Trip:
April UY: You would have to cancel no later than November to bank your points. That's fine, because you're likely to know whether or not you'll be going in Jan, by Nov.
Aug UY: You would have more time to cancel... Technically you could bank up thru March, but that isn't really useful. You can cancel as late as December. Gets you an extra month there.
Since you won't re-book for Feb-Sep, the later time on the expiration of August points doesn't really help you.
August Trip: Similar situation but even less pressure, since you can cancel up to 30-days out with either UY. In both cases, you'd have the option to rebook something for Jan or bank the points.
So there's not a huge difference in regards to banking.
More important to me would be whether you might ever want to go in the summer... When our kids were young, we did fall trips. But the bigger they get the harder it is to do things during the school year. So we never thought we'd want summer, but here we are a few years later and we will almost always be going in the summer from here on. If you might ever be in a similar situation, that April UY might be appreciated later on.
But then a benefit to the August UY is that if you do cancel an August or January trip, you would have 7-10 months to rebook if you could rebook for the summer. With an April UY, your only option would be to bank. So if you're ever working with points banked or borrowed then rebooking is more important than the banking deadline. We are always dealing with something banked or borrowed, so having a good window to rebook is more useful to us than to bank points from cancelled trips.
As you can see there are benefits to each UY, but they're all really situational.