To be fair you are looking at two very different cars. If it's reliability and economy you are looking for go for the 320D (they're still fun too if pushed hard).
Older RX8s are starting to show wear in the linings inside their rotary engines, an age old problem that Mazda has still not found a cure to despite years of trying. They are also quite thirsty beasts.
However...
Just look at an RX8 and tell me you don't want one....
They've got a certain something haven't they...
But I would go with the BMW every day. Make sure you look for a high spec (leather, trip computer, satnav, etc, etc) model in a desirable colour(i.e. black) with M-Sport body kit and 18" wheels. Also look out for cars that were bought with the 3 year service agreement (services free for first three years) as Irish dealers will honour this. That way you'll have a great looking car, you could save up to €10k on buying it here and you'll have a car you can use and enjoy without worries...