hi louisdave,
your question is similar to what i have thought many times
before. After reading and hearing abit, i conclude that
heaven is not same as pureland.
Western pureland is out of samsara, heavens are still in
samsara. Samsara means cycle of rebirth that continues
endlessly until we purify the ignorance that traps us in
samsara.
There are many heavens in the desire, form and formless realms
and in general we say that the causes to be reborn there are great
virtue/merit and for the form&formless realms' heavens, we say
that it requires very strong meditation power in the sense the one
is able to still the mind to a very deep level. After your
merit run out or after you come out of the meditation which lasts
for a very very long time, you still have to experience
rebirth.
Western pureland on the other hand is created by Buddha Amitabha
to save sentient beings from samsara. The causes for being
reborn there is strong aspiration or wish to do so, accumulating
merit, purifying negative karma, developing bodhichitta,
visualising Amitabha and the pureland. In Mahayana pureland
school, it is also said that just by wishing very much to go there
and reciting the name of Amitabha, one certainly will do so.
On being born there, one can be freed from samsara.
For certain very good practitioners who have realised emptiness
thoroughly, everywhere is pure to them and for them, there is no
coming nor going to a pureland. Thus to them, pure-land is
the nature of their mind because they have no preferences, even if
they shd descend to hell, it wld be like pureland to them. So
anyway, that is another connotation of pureland and it is an
experience beyond most peoples' reach.
I hope i have answered your question in a simple way. For more
info, you have to read up yourself more. If your question is
related to a comparative study between Christianity & Buddhism,
i have a source in mind that may be useful.