Actually, the MGS4 port wouldn't have been a great deal. Because, yes, it would be a year old port. And this MGS Rising spin-off thing coming to the 360 isn't such a great deal either. The 360 isn't "getting" the MGS franchise. Well, it is, but this game will have nothing to do with the kind of gameplay that's defined the series ever since its inception. You know what really would have been a great deal? MGS5.
Also, I have a feeling that Rising is going to be a very low-budget game in comparison with MGS4. The fact that's being announced just 12 months after the release of the latest MGS and the whole spin-off feeling that it gives make me think that this will be the case. Also, the fact that it's coming out on the 360 will probably mean that it won't have as many cinematics as MGS4.