By no means an expert, but much will depend on whether there is a particular use in mind, or for general use in a number of different circumstances. This might help narrow things down a bit
http://www.outdoorgb.com/sshop/tech_info.asp?ID=238. Basic advice would be:
1) Unless they are going to be used in conjunction with a stand, go for a lightweight compact set, so that the arms don't get tired when using them.
2) Work out some examples of what you want them for, and then work out how far away you will be viewing them from, and then how much detail you would want to see when magnified. Then you can translate this info into an example that you might see in the High Street, like car number plates, so that if you then go somewhere like Jessops, you cant try different pairs to see what suits best.
MG Mark