56's Man all the way.
Par 38's are great for the lamp cost, but for larger stages you are going to find that a Par 56 would be better. I started out with 16 (home made) Par 38 cans, they were great, I used them for years. When my gig's grew I noticed that the 150 watt Par 38 cans weren't bright enough for larger productions, I was out of luck if I needed to gel the lights (deeper colors, red, blue, etc) and still needed large output.
I really really suggest the 56's man. Start small with 300 watt lamps, maybe later on you're needs will change and you can upgrade to 500 watt lamps or DYS kits. Some things to consider before purchasing your par cans:
>Lamp Life: The life span of your par lamps will decrease if you are chasing or bumping your lights often. To that degree a 38 will be better because of the cost per lamp compared to the 56.
>Lamp cost: You can get a Par 38 lamp for less than five dollars, the cost for a 56 lamp starts at $15.
>Control: Figure out how you are going to get power to your cans. For stage shows with 38's I used a nice home made controller with 8 rotary dimmers that you can get from a hardware store. I also had about a million feet of extension cords which got really annoying really fast after tear down.
When I finally went with 56's I went DMX with a set of four DP-DMX 20L dimmer packs and a DMX operator controller. My set-up and tear down's go much much faster. Only one heavy BLACK 12/3 extension cord needs to go to the power source instead of 8-16 individual neon orange cords going to my DIY board.
>Power: You're 56's are going to need a lot more power than a set of 38's. I need four separate 20 Amp outlets to run 16 500 watt 56's. The par 38 setup was much easier on the electricity. Sometimes its annoying to have to run and find separate GROUNDED outlets. I am going to invest in a nice power distro soon.
>Fixture cost. Do some hard core online price shopping. You will find in some cases less than a $10 price difference between a par 56 and a 38. You can get a set of four 56's for $100. Ebay always has sets of six new for around $100!
Bottom line: Since I went to the larger light show with 12 56's, 4 46's (with DYS kits), and 4 64's I can no do larger (higher paying) gigs. If you do your pricing right, a nice Par 56/64 rig will pay for itself. You will be rockin' man!