I started off doing it subcutaneous 6 weeks ago. 1 CC hurted like hell every time, and left me with a very painful lump. Took 4-5 days for the pain to subside, and another week for the lump to dissipate completely.
Switching to half a CC twice per week made a big difference: much less pain (not half the pain, more like one eight of the pain), and only tiny bumps.
Then I switched to weekly IM shots at 1 CC. To be honest: so far those are by far the most tolerable. So I get the point about scarring, and that's not something I'm looking forward to, but at 1 injection per week (and rotating sites) I think I will opt for that from now on.
Now if I were using something water based, then it would be subcutaneous every time. My HCG dissipates almost instantly, and with no pain whatsoever.