Thought of going with PLs? A few twin PL-13 up-light fixtures can do a pretty good job of highlighting a tree at a fraction of the power consumption of either HPS or MH.
An added bonus is that you can change the color by simply changing the lamps from 27K (orange-ish, like an HPS) up to a 41K or even a 50K (white-ish, like a MH).
Maintenance and re-lamp costs are minimal when compared to an HID fixture as well.
Ruud has a nice fixture that mounts easily on a 3" PVC RAB mighty-post.
Good Luck!
