If I was considering a switchover to full frame within a year or so, I would probably have chosen another option than buying a new UWA for crop bodies only. The price drop from new to used is quite steep. A reasonable solution would be to buy a used 10-20 something . Then you could always sell it the day you no longer need it without too much loss. (With some luck you could even make a few bucks.)


The info about a full frame switchover in the relatively near future makes your decision even more difficult. It may be better to go for the telezoom first instead if you can live with the kit lens for wider angles than 24 mm. Your current 24-105 is a great range for full frame and you may be quite happy with the 24 mm at the wide end. Then it would be more useful to have e.g. a 70-200 zoom instead of more wide angle. (You would notice that 200 mm on full frame is not much tighter than 105 mm on your 50D.)


However, my most important advice would be to get lenses for the camera body you have, not for a body you may have in the future. Go for a used 10-20