There are several very good trainers on the market.

If you build your own:

PT-40 or Sig Kadet (there's 4 different Kadets, all are good The LT-40 is easiest to build)
OR... NEW, the "SPADET LC-40" will suprize most of the veteran instructors. It a Coroplast plane that looks and flys like the LT-40, not a kit... free plans, about as easy as you can get for construction. www.spadtothebone.com The plans are under the SPADWorld link. (detailed like any GREAT kit's instructions!)

If you want to go with an ARF... I'll again point you toward that SPADET. Its almost as easy to prepare for flight as some f the ARFs.
OR The Kadet LT-40 comes in ARF form
The RCM Trainer 40 is also excellent (If you coat the engine compartment with 2 hour epoxy. That will soak in and cure a potential poor engine mount joint, as it fuelproofs the area.)

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These are by no means the only good choices... these are some good options out of a long list of good trainers currently available.

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Don't forget to get tgether with an experienced R/C pilot. If your instructor helps choose the trainer, it'll be one he's familiar with. (better success rate... more accurate identification of potential problems.)

Also, your instructor can help you choose the best radio for your needs and your budget. You will want one that is "trainer cord" compatible with the instructor's. (4 major brands, 3 trainer cord systems... several off-brands, which are usually compatible with one major brand.) There are a LOT of good radios on the market.