Depends on your goals. Pidgeot and Beedrill aren't great end-game Pokemon, but if you don't have them in your Pokedex yet / are looking to catch them all, then you can evolve one of each just to have them. However, if your goal is to get the strongest Pokemon the fastest and don't care about collecting them all right now, then it's more efficient to just spend to get a bunch of Pidgey and Weedle to their second evolutions and not evolve them any further. It's not impossible to find Pidgeot and Beedrill in the wild, so you might not have to spend the 50 candy to fill out your Pokedex.
Given your plan to hit 20, which is a reasonable goal, I would just evolve to tier 2 first until you get 20 and then start stocking up candy to fill out your Pokedex.