.....I hope doing some 70.3 later, but work and three kids, limit my training time to around 10 hours pr week.
10hrs a week is plenty to do 70.3. Don't worry about the huge hours some on ST talk about training.
If you're doing structured power training indoors, then I'd definitely say the wheels are a better investment. The indoor training will help you improve bike fitness but will also really help you to calibrate RPE and HR against power, which you can then take across to your outdoor riding and racing.
I have both, and training with power has certainly helped me become a stronger cyclist, but I still find that on race day I pace primarily by RPE and use power/HR as a sense check more than anything else, certainly at sprint and Olympic distance. If you took my power meter away on race day I genuinely don't think it would have any impact on my performance, if you took my smart trainer away I'd be a slower athlete. And obviously race wheels are a guaranteed time saver regardless of how fit you are.
+1 to all of this
I like having the PM for reference but it's not my primary means of pacing up to 70.3 distance races. I didn't have one when I did IM but it may have been more helpful for that. I'd go with wheels first unless you needed the PM for indoor training.