I've never seen a 308 - but from what I can find it doesn't have spot metering capabilities (which would be one of the main reasons for using a hendheld meter over the in-built...) The 338 on the other hand do offer spot metering (higher price, but not nearly as high as the 508 or 608). If I had to choose between the 338 and the 508 I would choose the 508, but be warned - there's no rational reason for this! - it's just that I've tried the 508 and know that it does what I want from a light-meter.... I wouldn't advice you to rush off and get any of the models really - if you can find a shop that actually have them I suppose it would be a good idea to go there and try one out - just to get a feel for how this will work in practice. Will this work for you and your particular style of shooting? Will it take to long? Will you, after a while, get so annoyed with this nifty piece of electronics, that you place it in perpetual storage? I would also recommend that you try out the evaluative metering on your 20D - the partial metering isn't necessarily the best option in all situations. Let's say you are doing a candid shot, with the camera set to partial metering, and a guy with a white t-shirt walks into the middle of the picture when you press the trigger... If this one person fills some ten percent of the filmplane you will get a horribly underexposed picture, as the camera compensates to make the white t-shirt neutral gray... In this case you would most likely have gotten a correct result with evaluative metering.