All of these are in the price range you indicated, but there are other factors to consider to find the best fit for your temperment and present and future needs:
Ground Balancing: Do you mind manual GB or do you have to have auto GB? The Gold Bug II is the only one that is totally manual, the Gold Bug Pro is manual and it has a "Grab" button to GB it in one push. Manual GB requires more work to operate, but most experienced nugget hunters only operate in manual. The Whites GMT has a totally automatic mode, a manual mode, and a "Grab" button. The GMT gives the most choice in this respect to match your desired detecting style for a given area. The Tesoro Lobo Super Traq has a totally automatic GB, and matches the Gold Bug Pro in being the two easiest gold detectors to operate.
Other types of targets: The Gold Bug II and the GMT are only for prospecting because they only operate in "All Metal" mode, and they don't have discriminating circutry. The Lobo Super Traq and the Gold Bug Pro have Disc capabilities, and are probably very similar in operation and results. Which means you can take either of them to the park or anyplace else to look for coins and jewelry as well as the gold fields. The Gold Bug Pro has two factory coils, a 5" and a 11" DD coil. The Lobo Super Traq has nine accessory coils from 5 3/4" to 12"x10", to 3" x 18" and includes in the line up three concentric and six widescan(DD) coils . The GMT has three elliptical DD coils, a 3 1/2"x 6 1/2" "shooter" coil, a 10" and a 14" coil. The Gold Bug II has three elliptical coils also, a 6", 10", and a 14".
Visual Displays: The Gold Bug Pro and the GMT are the only two that have a visual display. They both show a numerical representation of where your ground balance is and the level of minerlization in the soil under the coil on seperate readouts. The GMT has another unique feature on it's display, an iron analyzer that through sucessive passes over the target, determines the probability in percent, that the target may be iron. The unwritten rule on this feature is if it reads 50% or below, dig! Some people dig everything anyway, but this feature just allows you to cover more ground if you choose to use it.