I find reaching the down tube location easier. Tradition says that the first cage goes on the down tube. In the '70s when I race, race bikes has a set there only.
All that said, I second Retro above. It's your bike. I'm good with whatever works for you, (I put a third bottle under the downtube for really hot and/or long rides. Time trialists use cages behind the seat, In the old days the pros had cages bolted to their handlebars. That trend is coming back. Camelbaks have become fairly popular on road cyclists.
Hydration is important, On hot or very hard rides, often of supreme importance. How you arrange your bottles and cages is far less so, as long as it works for you. I"m a pure traditionalist. I raced the two cages, down tube and seat tube (the seat tube cage secured with the metal straps), drank from the down tube bottom cage first, then swapped the bottles. But that's just me. I learned too long ago to change. You have a clean slate.
A consideration - will a Camelbak bottle stay in the cage if you hit rough road? Losing your only bottle on a very hot ride on a fast downhill on a group ride could be very costly; as in one of those nightmare you will always remember and hope to never repeat. I ride with standard bottles and their either "necks" of "waists" depending on bottle size in SS King cages because that combo never flies off.
Stay hydrated! Welcome to cycling! And Bike Forums!
Ben