|I like your reasoning. If I were to venture a hypothesis, it might be that in Life on Mars, 1973 was a good place for Sam Tyler, inspired by a childhood that was only blighted by issues with his father. That, at the end of the series, he consciously chose to go back there, would seem to show that, in some ways, he wants to be there - the initial catalyst being to save the others over saving himself, but he knows the implications of going back there 'permanently' - and '73, for Tyler, is the opportunity to effect real change to people like Hunt rather than the washed-out, corporate-style world he was stuck in in '06 / '07. In a lot of ways, it was like Heaven for him.|
For Alex Drake, however, '81 is a lot like hell; she is trapped there, believing she has full knowledge of the situation, and, worse, believing that she can get back, but taunted by the fear that all this is occurring in the seconds or moments before her death, and that however hard she fights to get back to '08 it may end up wasted, because she'll be shot through the head (as opposed to being in a coma in a hospital bed, like Tyler) in a grubby, unknown barge on the Thames.
So 1981 isn't a 'happy place' for Drake as 1973 was for Tyler; he apparently chose somewhere shaped on childhood memories of police shows warped around his own experiences, whereas Drake has - again, apparently - chosen 1981 for much more serious reasons, i.e. the opportunity to save her parents. The problem being that even if she succeeds in all her goals, what if all she's going back to is capital-d-Death?
All of which is why, if I had to guess, Ashes to Ashes has a broader, or more distinct, streak of fatalism than Life on Mars did; for the latter, it was an adventure with the aim of waking up; for the former, it's a quest with a very serious objective and no promise of a reward.