I personally would opt for an inexpensive, but decent second set of compatible (i.e. disk and cassette hub) wheels if the spare ones you have aren't. That way you have full redundancy in complete bikes ready to go and also wheels if you need it. You can certainly swap the "better" wheels to whatever bike you want to for occasions that warrant it, but you won't need to. You can grab whatever bike whenever you want. For example, maybe you prefer to ride one rather than the other if it's going to rain, and it is unexpectedly threatening - just grab it and go. if you're in a hurry, like you're late to a group ride and the bike you want to ride has a flat, just grab the other one and go or quickly put on the other wheels rather than fix the flat. Maybe you need to true an wheel but just aren't getting around to it. Etc. Even though it's not a big deal to swap wheels around every once in a while, you'll find it's more of a hassle at times than you want to put up with. For more than one reason, i's nice to have total redundancy. Plus the inexpensive but decent wheels are just fine compared to the nicer ones. JMHO, but you can find inexpensive, decent wheels easily for less than $400, probably closer to $200 used or close out.