Are you in a very busy area? Lots of neighbors, traffic, airplanes, etc.... ?!? I would say get quad shielded then like this being the price is pretty cheap; http://www.amazon.com/Black-Shield-Satellite-Broadband-Internet/dp/B00IBLW8BO/ref=sr_1_24?s=electronics&ie=UTF8&qid=1422296096&sr=1-24&keywords=coaxial+cable+rg6+quad  With a short run like 20 feet you don't really have to worry about anything else other than shielding.

If you had a very complicated, long run setup and a lot of splits for tvs etc.... you may have to look into distribution amps, lower gauge wire etc.... to mitigate some loss but in your case that's all not necessary since modems are on there own lines anyway and your at a very low length. Quad shielding will take care of any interference bleed you may be able to pickup if you were in a high traffic area with lots of cellphones, towers, electric lines, other routers/wifi etc...