I got a great deal on an Approach bag at 40% off (something like that) when Eddie Bauer was having a 40% storewide sale recently for the holidays. Haven't use it yet. My dog also has the Singletrack, which she's used multiple times. She started wearing the Singletrack around 10 or 11 months old, with nothing in it. Then I added my cell phone, keys and the doggie poop bag. Then finally some water (not all the way filled) before I put more.
One thing I don't like is you've to balance the two sides. If one side is heavier than the other, it will shift. Doesn't seem to bother my dog but it bothers me visually. With the bigger Approach bag, I'm told the problem will be exacerbated. But I want more room (for her to carry more of my stuff... haha) so will try it anyways.
If just a day hike, I think the Singletrack is excellent for that purpose. The dog also looks more approachable to strangers with a backpack on and less like a wild animal (my dog is a sable and goes off leash, people had mistaken it for coyotes since they are common here with signs all over that warn you to watch out for coyotes, quite funny).
Like others said, I'd not buy one till your dog approaches 1. At that age, I wore those fanny packs that also carry water bottles. Save your money for a good bag!