If its just the tax thats the issue and your happy with everything else, then go for it. I did and I don't regret it. If you want the benefit that a 2.5 V6 and Quattro system brings, then the slight downsides have to be endured.
I was shopping for a new daily earlier in the year, and I spent a fair bit of time comparing running costs. Tax varies so much between cars and I was tempted by some of the cheaper to tax options, but when I added reliability into the equation many vehicles had know faults, that would likely crop up and require money to sort. Any money saved would have been negated by the repair bills, plus reliability is very high on my priority list.
I was after a diesel auto estate, by a process of elimination I narrowed it down to two vehicles and went with a '56 plate Mercedes W211 E220 CDI auto estate (post facelift) (pre facelift have a few known faults, post facelift are reported as having all known issues sorted). The only catch was the tax, £260 a year, but as soon as you stick an auto box into the equation the Co2 jumps up. I don't like paying £260 a year, but thats the price I pay to have what I want.
Am I happy? Yes, every time I drive it, I know I made the right choice. It suits my usage, and is returning 40mpg on a commute into east London.
Good luck with your purchase.