Keep the X. Sell the Boxter and find a slightly cheaper 535i XDrive than the one currently in your budget.
..Because one simply don't sell an Imola Red X.
Jokes aside, considering that your Imola Red 06 is a relatively rare specimen, i would do as described above and hang on to the X for a while.
If it turned out that i liked the 535i so much that the X just collected dust in the garage, then -and only then i would sell that too and enjoy my 535i and what ever amount of extra monthly $$ i would have liberated in the process.