For me it just depends on how far I'm riding or more so...how much crap do I need to take with me.
For the occasional mountain blast I use a tankbag...the Joe Rocket Manta XL which holds a decent amount of stuff.
When I went to VIR last year and was staying for a long weekend I took the tankbag plus wore a backpack.
As far as having a preference...IF what I need with me will fit into the tankbag...that is my choice for sure. It stays in place and when I stop I can take it off the tank and carry it with me since it has a carry handle plus a shoulder strap. It also has a rainproof cover made into it...great product.
