1. What size hydration pack do I need?
This is affected by a couple of variables. First is the intended purpose of the pack. For running shorter distances then you'll want to look at a light, small profile pack with a moderate water capacity. You're not going to be carrying much gear, and you don't want to carry extra water weight. If you're using it for day hikes then you will probably want a pack with a large capacity gear pocket and a large water reservoir. Choosing a pack for cycling will be dependent on how long you plan to spend in the saddle. For cycling, you probably won't need much or any gear space, so focus on a pure hydration pack the size of which is dependent on how much water you think you'll need to carry.
2. What features do I want?
|The Reyes is a smaller pack|
with straps to allow greater
flexibility for carrying gear.
3. Do I want a hip belt?
My answer? Yes. This is up to you, though. Lighter smaller packs definitely don't need a hip belt. When you are carrying bigger hydration packs such as Hydrapak's Moro, which is a large day pack, you'll find that a hip belt to help support and stabilize is very necessary. Most come with sternum straps as well which is nice. Sternum straps keep the shoulder straps from slipping off and distribute the load more evenly on your shoulders.
4. Should I get an insulated tube?
A few last notes:
Remember your hydration pack will probably have lightly padded straps, you're not going to carry a lot of weight in it so don't worry. You can buy water reservoirs independently and put them in a pack that you already own, this might be a cheap flexible option for you. Look around for the cheapest pack that fits your needs, but I must recommend Hydrapak. I own both a Hydrapak (Moro) as well as a Camelbak and the Hydrapak is by far my favorite. It has become my primary day pack. Hydrapak's standard reservoir is also above and beyond easier to fill and use than Camelbak and their bite valves come with locking mechanisms so you don't accidentally leak water all over. It's a life saver.
Here's a review I did of the Hydrapak Moro.