The main differences i can remember between the field inferno that you shoot down at your local and the Terminator are that the terminator has a low pressure chamber and an expansion chamber which means you will get much more consistent gas pressure inside the gun, in varying conditions (cold weather particularly). Other benefits of the terminator are a velocity adjuster, double finger trigger frame, vertical feed, and milling on the body of the gun. Again, these are all i can remember...there might be more.
If you are looking for a decent budget marker....have a look at the spyder range
www.kingmanusa.com . I would suggest the E99, which i have, and so far can find nothing wrong with. It's also very cheap for an electronic marker! I think for around £200 you could do no better.