I wrote a long message which I seem to have deleted.
Pregnancy 1 - Bolton was local hospital, had all of my antenatal care there, couldn't fault it. Very good. Premature labour with twins - got to Bolton but they didn't have room in the Special Care Baby Unit so was rushed to St Marys (had never been there previously) and delivered there. Babies (and I) were there for 2.5 weeks, before we got transferred back to Bolton where we spent a further 6 weeks.
Pregnancy 2 - had moved to Manchester and although St Marys wasn't my local hospital, elected to deliver there. Had all my antenatal care there and baby. Straightforward delivery.
Verdict - both hospitals are excellent in my opinion. St Marys is incredibly busy, antenatal appointments always over ran, I was the last person to be admitted when I delivered 2nd time before the unit was shut (and ladies in labour were told to go to different hospitals - having said that, it may have changed since the new unit was opened). But if you can get past that, care is amazing. Special Care Unit - 100% second to none, I genuinely don't think I'd have my babies if I'd delivered anywhere else. Obviously hope you don't need any specialist care but you know at St Marys that you are in the best possible place in you do.
Bolton - babies were under their care for 2 years (as outpatients) so spent quite alot of time there. Again, very professional, good care. Community staff (eg midwife) brilliant.
I think if you think you're having a straightforward pregancy / delivery then Bolton is more local, logistics of parking etc are better, but St Marys probably edges it if you had to choose.