I have both and in IMHO they are pretty different. The Klon is really just a boost (a great one at that). I love it's simplicity and the magic that it adds to certain rigs. The LDO is an OD that has a lot more versatility w/it's 3 way switch. I don't know if I would choose one over the other. If you can swing it get both and then let your ears tell you which one you can't live w/out. If you're like me, you can't live w/out either of them and you'll keep them both. LOL. Good luck.
Correction: the Klon is an OD, but yes, a lot of people prefer to use it as a boost. Including me. But I also like to add in some OD gain and compression to taste when stacking. The gain knob, and the builders intent, make it more than and different from a boost. It's a magic color box!!
IMHO the Klon is a one-trick-pony (but what a pony!)
Klon is the King of Clean Boosts. It adds some grit if you want to but any more gain than 11-12 o'clock and the magic is gone. It's amazing at what it does and mine is nailed to my pedalboard but don't expect it to be half as versatile as a LDO (or MOD).
I tried removing my Klon from my board and I think that lasted all of about 30 minutes of rehearsal time. I keep it first in my chain (sometimes I put a fuzz in front of it) with the gain around 11 o'clock. I love it. I agree that it's one of the few pedals that does something no other pedal can do.
I just can't seem to replace it, though I could use the room on my board, the money in the bank...its just a clear boost for me - but taking it away feels like taking away part of my amplifier circuit. With it I can carry my little Goodsell Super17 to any gig, and either mike it or not - and sound wide and spacious and dialed in at the edge of things - with a tele or 335...what's that worth?