I'm surprised you are having to pay rrp. My understanding is that the rrp for the Domino is £545+VAT, but Ax and at least one other (can't remember who) had it recently for £512 inc VAT.
The Domino is a superb tool, doing loose tenon joinery quickly and accurately. But it is aimed at the pro, and it is difficult for us home woodies to justify that sort of expenditure.
What do you want it for? If you are going to use it, buy it and you won't be disappointed. But if it is a toy, you can do the same job with a little router and a jig or too. Not as fast, certainly, but just as well.
I have the use of one at the mo. It's not mine to keep, unfortunately. But it really does do what it says on the tin, and it isn't just a glorified BJ. For example, you can do face frames with it, and doing anything much narrower than about 70mm is out with a BJ unless you have the Porter Cable (I think) that does the FF size biscuits.
I didn't know about the Metabo either. I'd be interested in seeing it.