The 2015/2016 and 2017 are all F15’s so the year is trivial. A new X5 is probably coming out within the next 12-18 months too, so better to be in the short lease if you want to keep something new.
I personally like takeovers because of the short time frame, in this case you’ll also be able to miss out on paying 6 months of DMV registration.
If you’re all in at $774 with tax, the best you could probably consider doing is MAX $100 maybe $200 cheaper on a car like this. That’s $1800 or $3600 over the term of the lease.
If you like the x5 50i go for it (I had a 550i, but went to a 40e, and would not consider a 50i because I couldn’t deal with the terrible gas mileage). Only thing you have to look out for is getting hosed on the turn in.