Here are a couple things to keep in mind:
Porro vs Roof Prisms - As a rule of thumb, you have to pay about three times the price of a good porro prism set of bins to get equal performance from roof prisms. Porro prisms offer a lot of performance for your dollar. The down side is that porro prism bins are usually larger and heavier but compact models can be found.
Exit Pupil - A lot of people look at the objective size to determine light gathering ability. Objective size doesn't tell the whole story. Exit pupil is a better indicator. You can determine the exit pupil by dividing the objective size by the magnification. So a set of 8x40 bins would have an exit pupil of 5 (40/8=5).
The human pupil dilates to about 7-9mm in absolute darkness. The younger you are the more your pupil can dilate. The larger the exit pupil of your bins, the more you can maximizing the available light. So, if your pupils were dilated to 9mm and the exit pupil of your bins is 9mm, then you would theoretically be maximizing the available light. Anything more than 7-9mm is usually waisted. Of course there is light lost inside the bins so the math isn't perfect.
Based on my experience, I will not own a set of bins with a exit pupil smaller than 5. This will give you good dawn til dusk performance. Anything higher than 5 is gravy but bins with an exit pupil higher than 5 can be hard to find (7X50 would be an example).
Magnification - People often assume that more is better but are surprised that you can actually see more detail with lower power bins. Anything over 10X is too much magnification. In fact I find that anything over 8X is too much for me. The more magnification you have, the smaller your field of view will be and most importantly the more shaky things will appear through the bins. This shakiness can obscure details and fatigue the eyes quickly. I like 8X or 6X for general use.
Recommendations - The best value in all around bins is hands down the Leupold Yosemite (porro prism). I got mine for about $70 shipped but that was a good deal. They are more likely going to run about $90. The 6X is the pic of the litter. These are surprisingly compact, EXTREMELY clear, and fit a huge range of faces. Mine have never failed to impress anyone. They easily outperform bins I have that cost 3-4X as much.
If you want a very compact bin, the Nikon Travel Lights are good and the Bushnell Legends are EXCELLENT. Both are about as small as you will find for porro prism bins. Remember that compact bins will have smaller objective lenses so, in my opinion, lower magnification should be chosen - remember exit pupil!
If you have a bit more budget and want to step into some roof prisms, the Nikon Monarch line is an excellent value. I have their 8x42 and they are my all around favorite bins. I find myself reaching for these most often.
I recommend checking out an optics specific forum like OpticsTalk.com for lots of good info. Please don't hesitate to ask me a question via email or PM.
Good luck!
