I would think that they are BOTH very difficult, otherwise we'd have seen many female skaters doing them years ago. Not only that, but both are edge jumps, which require the skater to use pure brute strength (of the thigh muscle) to pull himself/herself up into the air, as opposed to the "toe" jumps (ala lutz, flip, toe) where the skater has the assistance of the toepick to launch himself/herself up into the air.
***Note: as regards upper body & lower body strength, both are needed - need the extremely powerful lower body strength of the thighs to launch oneself up into the air - and also need the upper body strength in order to stop the rotation.***
That said, I'd go with the 4S being a tad more difficult than the 3A.......purely on the basis that the former requires an extra 1/2 revolution........also triple axels have been officially landed (rarely, true) by four women thus far (aka Ito in 1988 @ Nationals, Harding in 1991 @ Nationals, Nakano & Nelidina in 2002 @ SA), whereas the quad salchow has only been officially landed once (Ando in 2002 @ JGPF). Furthermore, there have been more women officially attempting the 3A in competition (aka Onda, Asasda sisters, McCorkell, Nelidina, etc.) than the 4S (Ando; once by Cohen @ 2001 SA). 'nuff said. :D
Oops, one more thing, wanted to say that I've heard reports (for at least a year now) of Miki Ando attempting the 3A in practice. And that her success rate is around 60%. However, hasn't put it into competition yet. Easily understandable, especially as she has been attempting the 4S for only a couple of seasons now (landed her first unofficial one in the summer of 2002; landed her first "official" one the first time she tried it @ 2002 JGPF). Not to mention those various 3/3s (aka 3Z/3R, 3F/3T, 3T/3T) she has been attempting & landing since the 2002 season as well. Obviously this woman can jump, and has in her arsenal a nuclear bomb full of exotic jumps & exquisite combos, but is deciding on which ones to bring to the table. She keeps switching around (aka one competition she'll land the 4S & 3Z/3R, another competiton she'll land 3Z/3R & 3T/3T). IMHO she needs to decide by next season which ones she'll stay with & exclusively work just on those - my bet is her arsenal will eventually be as follows: open with the 3Z/3R, 4S, 2A (perhaps a 3A), 3F/3T, 3F, 3R, 2A.
***Note: IMHO the reason why Miki's 2A is average in height compared to the rest of her jumps is b/c she's been working on the quick revolutions more than the height. However, the distance covered over the ice is very very good.***
Peace & Love, Nadine (:^)
P.S. Have re-watched Miki Ando's FS to Firebird, and despite my carping on her presentation earlier, I've grown to absolutely LOVE IT (no lie)! Then again I am biased. (;^) But what has really blown me away was the monster jumping she displayed on the 3F (noticed it @ Jr. Worlds also) & the 3R! Without a doubt in the same league as Sebestyen, Onda, Slutskaya, & VV.
"People who never try never fall, but they never fly either." - M.F. Thomson