Darn good question... and one that's no so easy to answer for me. It depends on exactly what I'm doing. I have a vest and a chest pack... both loaded and ready to go.
Most rivers and streams that I fish don't require a lot of thought or very much gear. So, I'll use my chest pack most of the time. Occasionally, when I fish a more complex and bigger river, that may have a multitude of different hatches, I will use my vest because I can carry more stuff and a lot more flies.
So, I take a chest pack on my small streams and Lower Mt. Fork fishing trips and my vest when I fish big water like say... the San Juan River.
Now... I'm kinda like Terry. I have yet to find the ideal chest pack or the ideal vest. But, I'm still looking.
When it comes to chest packs, there is a lot to choose from. Again it depends on exactly what you need and to some extent what your budget will allow. I think the best value for a chest pack today is the Loon Outdoors Maxi Bandolero. For only $22 its hard to beat. The size is just right for most applications... but it can get pretty small pretty fast. The next chest pack that I'll get for myself will likely be the Fishpond Windwalker. I've tried some on at my shop and I really like it. But, it may be a little too big. However, that extra space can come in handy when I'm hiking a few miles to the creek and need to take a sandwich or a rain protection with me.
I'm almost due a new vest. When I get one, it will be the new William Joseph vest. That thing is super nice.
Anyway... Most of the time, I will opt for my chest pack.
But you know what else works???... That little purple bag that comes with a bottle of Crown Royal.