The trade off is ease of use (or use at all) versus weight, singles versus multitool, respectively. I have a crappy multitool at the moment, but I am looking at the Topeak single set (supplementing it of course) or the Park Tool MTB-3. Can't make up my mind either. Most of the time the tools are in their bag - they always weigh you down.

My current tool list:

•Bolts, 4mm stainless steel: Rack bolts break.
•Brake pads, AVID BB7: Figure about 3000 miles per set.
•Cables (brake & derailleur, caps): Derailleur cables occasionally do break on tour.
•Cassette lock ring driver: To facilitate replacement of drive-side spokes. http://www.jastein.com/PDF/cassette%20lock%20inst.pdf
•Chain checker: To test if one needs to replace the chain.
•Chain lube: Oil-based rather than water-based, for rain fastness. http://www.progoldmfr.com/products/prolink.html
•Loctite: Keeps bolts from unwinding themselves – it happens!
•Multi-tool: Hope that if you need one of its tools, the targeted fastener will be accessible.
•Patch kit: Make sure you bring a fresh one. Dried up cement does one no good.
•Pliers, small needle nose: For digging out wires that have given you flats.
•Power links: To repair a broken chain.
•Spokes & nipples / spoke wrench: Take a couple of each length.
•Tape, duct: Fixes a lot of stuff until proper repairs can be effected.
•Thermarest repair kit: For the NeoRest. http://cascadedesigns.com/therm-a-re...ir-kit/product
•Tire: A spare isn’t necessary if you start with new tires and / or your trip isn’t too long.
•Tire boot: Might keep a cut tire functional. http://www.parktool.com/products/det...17&item=TB%2D2
•Tire pump: http://www.topeak.com/products/Mini-Pumps/RoadMorphG
•Tubes: Two.
•Vise grips: To extract broken bolts from drop-outs and braze-ons.
•Zip / cable ties: Nylon 66 cable ties are incredibly strong. Require scissors to cut, not teeth.