I have just started on my second plane after my trainer. I used a World Models Super Frontier 40. It has a flat bottom wing so it may not be as advanced as the semi-symmetrical wing of the mach I and the Boomerang. However according to the "theory" the flat bottomed wing should allow you to progress to Solo faster than the semi-symmetrical wing. Again according to the same "theory" you will need a second plane sooner since you can not do advanced training with a flat bottom wing . I do not know personally if this is true since I can do all the basic aerobatics that have been mentioned previously in the beginner forum. (Loops, rolls, inverted flight etc)

According to my instructor at my field the Super Frontier 40 is much better quality than the mach I and the Boomerang, however I have seen then both at the field and I do not think, that especially if one takes into account that a trainer will be bashed about, that there is truly much difference in quality.

From my experience I would say the best thing to do is to go to your local field and ask one of the guys that will be instructing you which aircraft they recommend. Using a trainer that your instructor(s) is familiar and happy with will make them a lot happier to train you and hopefully mean that you progress faster.

I hope this helps

Cheers
Pups