The new wheel is significantly more aero with a 50 MM rim depth, so it's not just the tubular, it's more the aero thing...It's also just a really good deal.

I'd stick with the two Bonti's you have. As noted, you'd have to carry two repair kits. And what distance are you racing? Two carbon aero wheels saves very little time even on a half IM, so only racing on one-half of the equation really cuts into already small gains. So if you race sprints and Olympics, this sounds kinda silly. Plus, not having matching wheels will look silly. I've seen Zipp disks on the back with non-Zipp/non-carbon wheels up front, but having the front wheel be the odd-man out would look funny. Finally, IMHO, I wouldn't buy used carbon. Carbon doesn't bend or dent: it shatters.

Uh, don't forget that you can't just slap a Shimano cassette on a Campy wheel... You need to change the hub. But, since he already has Ultegra, he can use the same cassette and just switch the hub on the wheel. I'd get a shop to do this, because changing hubs really sucks. Hubs run around $60-70 for Ultegra depending on the number of gears...

You could also buy the whelset and a 9-speed, campy splined cassette, then change the spacing between the cogs. This is a bit of an annoyance, and requires some knowhow, but can be done. You may also need to readjust the derailleur for proper shifting, so you'd probably not want to do this for race wheels. It also costs a bit but could be cheaper than rebuilding the rear wheel. I did this on a set of x-lite tubulars with a campy free-hub and am happily, and effectively, running 10-speed SRAM on it. Also, depending on the wheels you may be able to just swap out the freehub on the rear. What model and year are you looking at?

Ans as for not having a complete wheelset, who cares? I'm running an x-lab 3 spoke on the rear and Aeolus 6.5 on the front (minus the vile Aeolus decals) and it looks just fine. Sure the carbon doesn't match perfectly and the stickers don't advertise my sponsorships (or how much money I wasted on dimples to get an extra 7 seconds in 40k), but I assure you I don't ride any slower due to the mis-match. And there's the added benefit of being race legal in any non-draft event.

And the dual repair kit can be solved by taking sealant ad CO2 only. It'll work just fine with either tubulars or clinchers (Vittoria Pit Stop is a solid choice, but you'll still need to use your CO2 afterwards).

i'm 36 and dropping $2k for wheels doesn't make a lot of sense either :)
I'd have an easier time dropping 25#, and i'm not fat by any means...

I got my 3 spoke rear orr ebay for less than 400 and the Aeolus front for about 500, but new. Complete with Tufos and skewers, I spent less than 1k. As for spending 2k on wheels, who seriously does this? Join a club, most of which can get you discounts, even on Zipps or Xentis. Or get them off ebay. Or, and this is the most important thing about race wheels, get fast enough to get a pro deal and buy them at cost (should be about half retail). If you're considering aero wheels, you should be doing it because they will make a difference whether you qualify for worlds or reach the podium. And if you do either of these at 23, you're probably good enough to get sponsored. Otherwise, stick with your x-lites. They're solid wheels, climb well, and will be fast enough to race on until you get that pro deal.