Whatever you use, be sure to seal the underside and endgrain as well. If you only seal the top, you risk moisture intrusion into the bottom from humidity and you will end up with a nicely curved top.
I use mineral oil for food prep surfaces. It is easy to apply, cheap and odorless. However, non-drying oils basically protect wood by filling its pores and thus preventing moisture and other substances from getting into the wood. These finishes should be renewed often to keep the wood saturated otherwise you are very likely to stain the benchtop with glue, blood, oil, grease, coffee, and anything else you manage to get on it.
Drying top-coat finishes protect the wood by forming a hard protective layer on top of the wood's surface. Many of these finishes are also resistant to chemicals, acids, and UV rays (i.e. spar varnish). Hard finishes hold up well to abrasion but can be compromised if you cut through them with something sharp. If you can afford the drying time, and have plenty of ventilation, Spar varnish is not a bad choice. I would go with a satin finish because you do not want your bench constantly reflecting light into your eyes like you would get with a high-gloss finish. Just let the top get scratched up for a few years until it bothers you. Then sand out the scratches and add another coat.
