What is your budget?
How many axis do you need 2, 3, 4?
Glass scales or magnetic scales?
Numerical number display or graphical display, do you need advance functions such as bolt circles, arrays, tool datum libraries?
Many people these days are gravitating toward Yuri's Touch DRO which works with a wide array of scales, and you provide your own touch screen PDA. It will also integrate with some battery powered spindle mounted scales. There are inexpensive 3 and 4 scale DRO's, general recommendation is to stick with glass scales due to poor quality of the magnetic scales in these cheaper DRO's. Better quality DRO's (read $800+) with magnetic scales work well, AKA Electronica brand EL700, MagXact Magnetic MX-200, Easson 12B/C (Machine DRO UK), and probably Aikron. There is a ton of previous posts in these DRO's and reviews.
www.yuriystoys.com This forum is the home of TouchDRO on Hobby Machinist. Yuriy's products are DIY DRO projects, plans and software that you can use to bring DRO to your hobby machine shop.
www.hobby-machinist.com