I have a set of DB5's on my bike currently; they are under powered (180/160mm rotors) and have a horrible spongy feel.
Plan A is to:
keep the DB5's upgrade the rotor sizes, new pads and a re-bled this will cost about £110
Plan B is to:
fit the SLX's, this will require a mismatch mount, new reverb lever and a bled to get hose through frame (possibly new hose too) this will cost about £120
which would you do? never been amazed by the DB5's but loved the SLX's when they were on my bike about a year ago. I have no idea whether they will need a bleed now though.
I have a set of DB5's as well and while they don't feel spongy they certainly feel underpowered. I think if you upgrade the DB5 you'll still have issues where as with SLX that might be the end of it. So my thoughts are to go SLX.
SLX no brainer.