Good points, guys... a .25 isn't inherently harder to fly. In fact many people learn in Hong Kong on a .15 size WM trainer that is not offered elsewhere. However, my recommendation has always been to look at what you want to be flying next. By looking one step ahead, you can prevent yourself a lot of anguish. Since you know that you will want to be flying that plane Slow Poke soon and you may not want to invest in another set of radio gear and another engine, you may want to get a 25 size trainer so that you can transplant all that equipment once you've mastered (dangerous word to use) your trainer.

In terms of flying, I feel like the .25 size trainers have a higher wing loading and tend to fly faster probably due to using the standard radio gear. 40 size trainers just seem to be able to stop in mid-air. As ELTIGRE mentioned, this could be a good or bad thing! Anyway, you could always clip the wing of your trainer but hard to add more wing area to it!!!

Furthermore, most people that start in this hobby don't already have a plane so I would still recommend a 40 class plane because it offers the most choices in terms of transplantability. While it seems like the selection of 60 class planes is growing rapidly, the 40 class planes still seem to be the mainstay of the industry. As such, going in on a 40 class trainer means that you have many more choices for a second plane.

Back to the beginning - I would say that the perfect engine for that slowpoke is a Saito 40