I like to use both - I went on a ride christmas day and went through two spare tubes and 3 CO2 carts and then I was done and had to SAG home. CO2 won't leak out of the tube during the course of a normal ride, but you will want to empty the tire and re-inflate it with normal air when you get home.

I have a Road Morph G and that's a great pump to use but a PITA to mount on a normal road frame. I kept hitting my leg with it (had it kind of snugged in between the seat tube and the bottle mount). The plus is that it's a very easy pump to use on flat tires. That gauge is very difficult to read though.

Now I have a lezyne road drive, which takes up no room at all but I'm sure is harder to use just by the nature of its size.

As an aside, a small pump also makes finding the leak a lot easier. You CAN blow into the tube but IMO it doesn't make finding the leak very easy.

So, the TLDR version - both are useful.