Which one ?
Wheels make a huge difference. Not only should the Zipps be a lot faster they should also make riding your bike more enjoyable. If you don't have Di yet you won't miss it. If by triathlon bike you mean TT and do the flatter faster courses how many times are you going to change gear? Assume you will also get an aero helmet
Di2 is awesome. And you can get a decent set of carbon wheels for much less than Zipps these days.
Di2 is much more of an advantage on a TT bike than a road bike. Ultimately you will end up with Di2 and a set of decent wheels, if not now then later.
Have a look at Walker Brothers wheels - less than £1000 a set, they are as fast as Zipps apparently (I just got a set of second hand ones - done less than 100 miles)
A Di2 upgrade can be done for £500-£600 if you buy bits on ebay (I did this over the winter)
Much easier and (probably) cheaper to upgrade wheels later on, Di2 you would probably need to go to your LBS for it.I would go with Di2 and then for the extra money you could pick up some Flo or Hunt wheels or something a be pretty close.
Wheels you can also choose what to buy based on budget, sales etc whereas Di2 you will always be looking for the same components, then have to make sure you get all the right bits etc or you end up paying your LBS to do it. Unless the wheels are Zipp NSWs then I think its a no brainer to buy the better drive train and upgrade the wheels at the right time for you with little/ no hassle.