I already have many standard power tools (reciprocating saw, jig saw, miter saw, circular saw, drill and impact driver, angle grinder, air compressor and nailers...).

Things I thought I might use these types of tools for are:
- cutting a square hole out of hardwood floor for an air duct (although nails that aren't visible through the tongue & groove could make this tricky)
- cutting sheet metal ducts and gutter downspouts
- cutting notches out of tile for backsplash going around back of sink
- small routing jobs (hinge mortises)
- cutting holes in drywall
- detail sanding
- cutting existing trim and baseboard shorter to install a cabinet

If I can only buy just one, which one would you buy and why?

For what you want.
- cutting a square hole out of hardwood floor for an air duct (although nails that aren't visible through the tongue & groove could make this tricky)

Fien tool - the rotozip struggled cutting through hardwood - lots of burning and went very slow - actually I don't think either it a great tool for this job- a good quality jigsaw would be better.

- cutting sheet metal ducts and gutter downspouts
I have not done a lot of metal cutting with either - but I think the oscelating blade would be better. We've done most of that with a metal blade on the compound miter saw -larger projects get cut with a blade backwards in a skill saw - or by hand - bought a nibbler but did not like the way it works (this was for work on outbuildings).

- cutting notches out of tile for backsplash going around back of sink

I found the rotozip worked wonderful for this (need the right bit) - put your tile on a piece of scrap sheetrock so you can keep the whole tile supported (just cut into the sheetrock). Was worth the full purchase price even if I never use it for anything else.

- small routing jobs (hinge mortises)
I would not use the rotozip - it is harder to control than the Fien tool and you really don't want to slip doing this work. For these things DH uses a good wood chisel. Bigger routing jobs get done with the router.

- cutting holes in drywall
Rotozip made way too much dust - best here is a hand saw of some sort. Utility knife and a steady hand does not make near the mess. For small cuts (like outlets) an exacto knife with a 3" serrated blade was our favorite, but we have not found blades like that again. It was pointed enough you could push the blade through the sheetrock and then start cutting.

- detail sanding
Unless you have holes and need a drum type, the oscelating tool wins hands down. However I've got several nice sanders and they work better than the Fien tool for the job. I also have drum sanders for my drill press so I've not had to use the rotozip to sand anything -

- cutting existing trim and baseboard shorter to install a cabinet
Fien tool hands down. Main reason we bought it. There have been so many times in our whole house remodel that the oscelating tool has turned out to be the ONLY tool suitable for the job.

BTW - I second what others have said - dremel is great for the hobbiest (woodworking, arts & crafts) but I've never considered it for construction type house work. I owned one years ago but gave it away at some point.

Cathy

drill, then jigsaw or sawzall (btw, those Ridid Fuego's look tempting) - the multi-blades would not hold up for this.

---------
- cutting sheet metal ducts and gutter downspouts
sawzall w/ right blade, but the multitool is great for some odd cuts (eg. hole into the gutter, etc)

-----

- cutting notches out of tile for backsplash going around back of sink - dremel or dremel Trio

------
- small routing jobs (hinge mortises)
do it by hand, or trim router (a great tool), or maybe the new dremel trio. I'd go with the trim router.

------
- cutting holes in drywall
multi-tool (less dust - catch it with a shop-vac)
-----

- detail sanding
it depends

-----

- cutting existing trim and baseboard shorter to install a cabinet
two thumbs up for the multi-tool - worth it for this function alone

Homebound, I meant the standard dremel (not the dremel multi-tool or the new Trio). I'd be interested to hear any reviews of the new Trio, but I think I'm less likely to buy it than an oscillating multi-tool.

Thanks again!