Which camera bag should I buy?

I'm looking for advice on a camera bag.

I've looked at the reviews of quite a lot of them, but I still can't point my finger at any bag that will be the best fit for my needs.

I've looked at a lot of different types of bags (shoulder, back and sling) but I just can't figure out which one will be the most useful for me.

What do I need it for?

I have a daughter that rides horses, so 95% of all pictures are of an equestrian nature from showjumping and dressage 

competitions.

My old camera (or current until my Mark III arrives) is a Nikon D70s with a 70-300mm lens.

I usually have my kit lens with me in the car, but find that I rarely switch lenses.

My tripod is usually in the back of my car and I find that I don't use it very often.

My thoughts about the different types of camera bags

When I started getting more and more into photography and started "wearing" my camera everywhere I went I played with the idea of getting a sling bag. I liked the idea of having the camera safely tugged away, but with fast access.

But when I decided to get the 5D Mark III with the 70-200mm IS II USM I kinda left that idea behind thinking that I couldn't find a sling bag that would fit this combo.

Then I started looking at backpacks and shoulder bags - and I'm still torn between the two.

Backpacks are convenient to carry around and with plenty of room.

I don't see myself doing any trekking with my gear, but when we're out at competition I sometimes need to handle the horse and I'd rather not do that with my camera wedged between myself and a horse. So it would be nice with a bag nearby to safely tug my camera away for the next 10 minutes.

I will almost always be near my car, so that left me thinking about the type of bag that I never thought I would buy - one of those square shoulderbags.

90% of the time I'll be using the bag as a storage unit where all my gear can sit securely while it gets transported to and from competitions, but then I'll be missing out on the option to tug my camera away in a jiffy.

So I think the best bag for my needs will be a backpack.

And now the big question:

Which one should I get?

I don't have a lot of gear and don't really plan on carrying multiple bodies and multiple lenses with me.

In essence I just need something to safely transport my gear in the back of a car which I can also carry around for safekeeping my gear whenever I need to handle a horse.

The perfect bag would be large enough to hold a 5D Mark III and the 70-200mm with the hood attached

Any ideas?