My dad's old pack has finally given out (like an 1970 external frame... man did it kill on long hikes) and i'm looking for a new one, but this is my first time shopping for one. Should i go retail or online? Where are some good places to shop? And i know for sure i'm going for a internal frame but i'm not sure on the size, i'm doing a thru hike next year. Should i go to a retailer and check sizing and then go online for deals? Are there any other features that i'm missing? Any help would be great, Thanks.