Like the title says, if I replace the rear shocks which of these would be the best choice (car is a 2003 SDI).

OEM from dealer £42 inc VAT

Bilstein B4 from ebay £34 inc VAT

I think the standard ones are oil filled and the Bilstein are gas, I know from past experience when I bought some cheap shocks they wore out in 6 months! I have also bought Monroe gas shocks and they were very good (but expensive). Are the bilstein a step up and will they last as long as the OEM?

Moving up to the next level to Koni / Monroe appears to double the price which I cannot really justify.

I've sort of decided myself that the Bilsteins are a bargain so will probably get some of those. The OEM ones are oil filled.