I don't know what computrainers cost, but...if you already have an SL setup, then all you need is a spare rear wheel and a receiver. You can use the same CPU.
Further, since it's a trainer wheel, you don't care how heavy it is...so no need to get another SL, you can get the standard/pro hub. Another benefit here is that, if and when the SL has problems, you have a spare training wheel for the week or two it's gone getting serviced.
You have a much higher probability of the powermeters being consistent with each other if you use two power taps, as opposed to a powertap and a computrainer. Since you're using cyclingpeaks, I assume that consistency is important to you.
I'd consider the trainer upgrade a secondary issue, that doesn't stop you from getting the second PT hub/wheel. If I had money to burn I think the KK Pro (with the add-on weight to make an 18lb flywheel) would be great to have, but I don't ride the trainer so much that the $300 - $400 isn't better spent elsewhere.
I can't think of an argument in support of the computrainer.