From this web:
http://hk.myblog.yahoo.com/josephsit/article?mid=25
you can compare both school's bandings in HK.
Well, St Paul's College is of band 1-2
From the folling website, you can take a look at how different people comment on St Paul's college..
http://forum.edu-kingdom.com/viewthread.php?tid=1103963
-"有人話band 1, 有人話band 2"
-''同埋個banding 咁wide, students由band 1到band 2尾都有....''
while 聖心 is definitely of band 1
You can also address to HK EDB for more info.
and comparing the the conduct of 聖心 and St Pauls college, 聖心 is definitely better.
SO, all in all, i think 聖心 is better