I have flown both though the rex was an SA model. The biggest difference I found was while the 400 will fly out of the box it does need to be looked over to double check pitch settings and such. when you buy a 450 you have to assemble it from scratch. You will learn more that way and it garanties your setup is correct as long as you know what you are doing. As for the DX7, I have one but I would have stuck with the DX6i except I want to go nitro in the future and will need the seventh channel to set up a govenor. Buy the way, the DX6i is easier to program IMO. Best of luck with either choice.

Almost forgot You can get the DX7 as a package with I beleive DS285's and an AR6100. Both excelent for the 450.