Whatever bag you choose, I strongly encourage you to have nothing larger than a carry on. Carry-ons are much easier for trains and buses in addition to planes. You can carry an inordinate amount of clothes/stuff with a wheeled carry on + a medium size back pack as your personal item. You will probably want a backpack when you get to where you are going anyway.

Don't forget, you can wash clothes as you travel (check this board for clothing suggestions), you can pack very light and buy clothes as you travel which make great souvenirs and if you buy too much, you can always ship some stuff home.