I'm confused by some of your terms -- "or" ... "too many bikes" .... I think what you meant to ask is "How do I fit more bikes into the space I have available?"

Otherwise, it would depend on which one I enjoyed riding more. If I really couldn't discern a significant difference in the ride I'd choose based on color. Is your Ironman yellow or green? If green, I'd definitely keep the Tempo. The fact that your wife and son have Ironmen could tip the scales one way or the other depending on how you and your family feel about homogeneity. My family values diversity so they'd be put off if I kept the bike to be like them. Other people I know like to have matching bikes.