If you were a WHL general manager and had a top level prospect drafted in the bantam draft, for his 15 year old year - where would you rather he played? Junior A, Junior B, or Midget AAA. Keep in mind that this is the WHL so those players would most likely be eligible for the leagues in B.C., Alberta, Saskatchewan, and Manitoba - therefore, arguably the better of the Junior A leagues for developing players. Now at the same time, if the player enjoyes playing Junior A, there is also the chance he could snub your team and go play NCAA. Other factors that must of course be taken into account are ice time and coaching level.
Depends on where he's from, to begin with. If he's got a good midget team in his hometown, probably that, since he's going to get the most playing time there. If that isn't the case - or if he's so good that he can get a regular Junior B shift at 15, like guys like Kerr and Gillies are doing in North Delta right now - then Junior B. Don't think he can play full-time in Junior A at 15, so that's not an option - not that it'd be a good idea, anyhow, because of limited ice time and, like you said, the possibility of him getting ideas about getting on the college track.
The most important thing is that they get enough ice time. That being said it depends in which province the player lives.
In BC the Midget programs are traditionally weaker than in the other provinces (generally speaking), so if they can get a regular shift there, Junior B would be the better option. Of course if they are a prospect like Gilbert Brule, Junior A also is an option.
In Alberta the Midget programs are strong, but Junior Bs are more like beer leagues, so they are no options for 15 years old. I think some Minor Hockey Associations in the province (Northwest in Calgary for example) still forbid 15 years old to play Midget. So a guy like Devan Dubnyk was forced to play another year bantam hockey after being drafted by the Blazers. On the other hand a kid like Dan Blackburn played Junior A as a 14 years old in his hometown Canmore and it worked for him (I think his Midget AAA team would have been in Strathmore, a two hours drive away).
Bottom line is it depends on the player and his situation, to decide what is the best option for him.
In BC the Midget programs are traditionally weaker than in the other provinces (generally speaking), so if they can get a regular shift there, Junior B would be the better option.
That's changing a little, though, with the new BC major midget league. It didn't stop the top 15-year-olds at the coast from moving to the PIJHL this year, but if it works out in the long run it should make midget a better option in BC.
In Saskatchewan, we have a great Midget AAA program --- 12 strong programs spread throughout the province. Therefore, the SJHL is often not an avenue that is used by young players mostly because of the ice time which would be available to them. At the same time, this also hurts the league in recruiting the top 18 year old players who traditionally would still struggle to receive the ice time they could very easily get in the AJHL and BCHL. This mostly has to do with the average ages of the leagues (traditional # of 19 and 20 year olds). However, this is not reflective of the level of play. One big influence is the likelihood of a scholarship, where recently (IMO) the SJHL has fallen behind.
As for Manitoba, there is a joke running around Saskatchewan that the MJHL is a glorified beer league itself. The Anavet Cup, presented to the team who wins the series between the two leagues to advance to the Royal Bank Cup, has been presented to an SJHL team 23 out of the last 30 years. Therefore, it cycles down through the other leagues. As a result, many times a 15 or 16 year old player will have been rushed to the MJHL rather than play Midget because of the lower level of play in the league. Not to say there aren't good Midget programs in Manitoba (ie. Winnipeg Wild, Brandon Wheat Kings), but it is just a matter of fact. In the last year or so, the level of play in the MJ, itself, has made progress in national competitiveness. Yet it hasn't reached the point where it's producing a threat to take the Royal Bank Cup year after year.
Therefore, Manitoba is a bit of a juggernaut in that out of all the Western provinces, it is the most difficult to predict where an elite 15 year old player will go. One recent example is of course Jonathan Toews, who went to Shattuck's St. Mary's in Minnesota rather than play in the MMAAAHL or the MJHL. In the opinion of his family and his agent, going to Shattuck's was the best for advancing Jonathan Toews' career. He, of course, snubbed the WHL --- but that's argument for another thread (ie. which is the better option - WHL or NCAA)