LOL you are going to have to decide that for yourself, cause everyone will have a different answer! For a small 2 plant grow one thing is for sure you won't need massive amounts of light. However light is everything in this hobby, I always say, so if you don't invest in some quality light then don't be mad when a meager crop comes on. So now onto my suggestion.....I too grow 1-2 plants at a time. For vegging a I use CFL's cause they are cheap and plentiful. I probably have 200-300 actual watts of cfl on it before I flower. Then I get rid of the CFL's and bring in this:
http://kessil.com/horticulture/H350.php
I find that one of these will handle two plants easily. The intensity of light is comparable to a 400 watt HPS or MH. However the advantage is that there are no hot temps, ballast, hoods, venting, it just goes on and on. I can leave it on 20 hours a day and not worry about my house burning down (yes I'm paranoid...it's the weed). Now some will argue that $250 for one of these is steep, but I think you get what you pay for? Yes you can struggle with a 150 watt HPS hood, bulb, and ballast for $75-100, which is half the money, and get the same job done. I prefer something that is small, light, and easy to adjust. Now that I have two of these lights. I use a purple to Veg two plants and both two flower.