I've worked in both countries, and they are both quite OK. Far better than just about all the sub-Saharan countries.
Libya is probably the better of the two, it is for the most part a rural, agricultural country, and the people are very pleasant. If you have a relaxed nature and can 'go with the flow', you will probably make friends and enjoy your time there.
As others have mentioned, there is a bit more activity and perhaps a few more choices in Algeria (you can buy wine, beer, etc.), but there are more people there, more political dissent, etc. The commercial atmosphere amongst the foreign resource companies is more competitive (there are far more foreign companies in Algeria than in Libya), and this can sometimes create a bit of tension that you won't find in Libya.
Both countries can be unpleasant places to be during the heat of the summer, and also during fly season, but for the most part, the climate is pretty good.
Zimex is a good company, I have worked for them in the past and have pleasant memories.