I can't agree more with the absolutely supreme utility of a locking tonneau in combination with a locking tailgate. Invaluable for road tripping with a load that won't fit elsewhere.
I have a RETRAX hard roll-up cover & I absolutely love it (for 10 1/2 years now). I WILL make one observation for anyone considering the various forms of bed covers: I was just in Utah where we got some good snow (8-10"). After of few days of being shut, I discovered that I was not able (without an amount of force I was unwilling to apply) to get the Retrax to roll-up. It was just frozen up. I'm thinking the tri-fold types would not present this kind of issue if you live in cold weather climates.
But by the same token, I DON'T live in these climates, and it's possible there's a simple solution to this problem (with the RETRAX) that I'm just not aware of.