1:49 p.m. on November 6, 2006 (EST)
As Steve indicated, for something like a Philmont expedition, you will find an external frame Kelty to perform much more satisfactorily than any internal frame. You are on good trails all the time, except for short side trips (you don't carry the pack on those, anyway).
Yeah, yeah, I know, internal frame packs are all the rage and THE fashion statement these days. But you want comfort, good ventilation (it gets hot on the trail in Philmont in the summer), and when you make the little side journeys to get the next food pickup, with the external, you just strap it onto the frame - none of the repacking nonsense, and no hard, odd-shaped object poking you in the back. Besides, externals are much cheaper for the same quality than internals (like half to 2/3 the price for the same capacity).
I have and use both internals and externals. Each has its place (as do frameless summit packs and daypacks). Having hiked Philmont trails a few times (I was there in Sept for a workshop that based at 'Cito), and having a son who was there as a Ranger, externals are most suitable for the trails and terrain you will encounter. Internals are better for off-trail, backcountry skiing and snowshoeing, climbing, that sort of thing. They are warmer (because they hug your back closely), and that means much hotter and sweatier in the hot conditions you will encounter in much of your expedition. So, for example, I used an internal (Dana Terraplane) on Denali, Ixtacihuatl, Orizaba, Rainier, and similar peaks, and for multiday ski traverses in the Sierra, Tetons, Rockies, and elsewhere. But I often use an external when humping a load of climbing gear to a basecamp in the Sierra or Rockies backcountry, where I am on trail, but have to haul 60-80 pounds of ropes and hardware. Once at the basecamp, I switch to a light summit pack for the climbs themselves.
As Steve said, you can also get used packs from many sources, including your neighborhood Goodwill.
On seward's list of favorite and unfavorite brands, packs are like shoes - it is vital to get a good fit, especially for something like a Philmont expedition, where you are hauling a load for 10 days for 55 (shortest treks) to 80 (longest, "Super Strenuous") miles. Like shoes and boots which are made on different lasts (that's what the model foot is called that footgear makers use for the footgear design), packs are made to fit some "ideal", "average" body, sometimes sized. If the boot last matches yours, the boot will be comfortable after 10 days of hiking. Same with the pack - if the maker's "ideal" body matches yours, the pack will be easy to carry for the weight. But if you don't match, you will suffer miserably for the whole trek, and maybe want to give up after only a couple days (major piece of advice - find a shop with an experienced pack fitter, which leaves out most REI's and EMS's, and makes mail order problematic). One of the other advantages of external frame packs is that they are adjustable for back fit much more than internals.
I find that I fit Dana and Osprey internals very well, but I have yet to find a Gregory that is comfortable. Friends of mine find the opposite. So while it's fine to take people's advice on quality of construction, you have to fit the pack personally, because my body shape, and seward's, and Tom D's, and Ed G's are all different, and probably don't match yours.
By the way, I have, and use, a Kelty Cloud, and find it an excellent pack. The Red Cloud is similar.
I would also suggest that you think hard about what you will be carrying at Philmont. Almost all crews take way too much gear. At your Ranger inspection the day before you set out on your trek, the Ranger will have you lay out all your gear, then review it for completeness on the one hand and excess on the other. Virtually everyone will be told to lose at least a quarter of their gear, and many up to half. Most people will also be told to head for the shop and pick up one or two vital items. It still will not be "light-packing" (and far from "Ultralight"). But, my advice here is think really hard about what you will really need.
And print this off and show it to your adult crew advisors. Point out that this commentary comes from someone who has been to Philmont as an advisor and has a son who was a Ranger there. So it is based on real-life experience with Philmont treks.
In picking your expedition, consider strongly picking one for the staffed camps (that is, the activities), and not to try to set Personal Records for distance and climb. The activities are what really make Philmont a unique experience. Everyone I know who has done one or more expeditions has remembered the activities as the number one thing. The troop I used to be SM of does Philmont every other year (except for a few Northern Tier treks), and I work at Council level in a council that has at least 4 or 5 Philmont treks every summer.