On the other hand I am not sure it is truly worth the expense.
Let me pre-empt this with an apology, but I'm grumpy today.
"Worth the expense"--worth is it's value to you, which we cannot define, and relative expense is hard to determine without having your financial statement in front of us. If you need (or want) a body that does more than one you currently have and have the funds for it, then go for it.
At this point the 5d3 is completely theoretical, so trying to compare the two is not doable. You can certainly hedge your bet by not pre-ordering the 1DX with the hopes that the 5d3 will at least be announced before the 1DX comes to market and you can then make an informed decision between the two. But this is at the risk of "going to the back of the line" for the 1DX.
Good luck with the decision!