Marcus,
IMO, the C12 with chuck kit is the exactly right drill for that. I bought mine
specifically for working on my joists. The short head fits between the joists and studs. Many times you can drill holes using the C12 alone with no chuck. 1/4" bits fit (loosely) directly in the head with no chuck. Sometimes you will need to use the Festool right-angle chuck for the really tight spots. With the C12, you have multiple options and can use one hand in many cases.
If you look at other drills, the head is WAY longer than the C12's head and that includes the TDK. The Festool right-angle chuck fits both tools so you could use the TDK too. However, the downside of the right-angle chuck is that you need two hands to use it.
Take a look at the pics at the bottom of my Depth Stop Chuck review thread:
http://festoolownersgroup.com/index.php?topic=331.0. One pic shows my joists with lag screws pre drilled. The fuzzy brown thing running between the joists is a 1/2" copper water pipe. In the background, you'll see a white power wire hanging down. That was then.
Now, months later, the piple turns right and goes through three sistered joists between 3" and 4.5" thick (double sistered). The white wire goes to the left through two sistered joists. I used the C12 with 3/4", 1" and 1-1/4" spade bits to drill the holes through the fir and Microllam joists. FYI, Microllam is MUCH tougher to drill than standard wood joists. Drilling a 1" hold through a single joist is nothing.
One strong recommendation... Use the Lee Valley Speed Bore spade bits:
http://www.leevalley.com/wood/page.aspx?c=2&p=32238&cat=1,180,42240,53317 for this kind of workl. They are relatively inexpensive and work very well. Their only downside is that they only go up to 1". They come in a standard length shaft. Other companies offer extension shafts that work fairly well and you can get some short shaft spade bits for the really tight spots.
Also, if you're doing a lot of holes, get at least two 3/4" and 1" bits in each size. The probability of hitting a hidden nail head is pretty high and then the bit is toast.
Good luck with your decision.
Regards,
Dan.