I know all about the rain
uhh probably not - I like tents too.
for the changing parts
Bivi and Tarp - 1140g - less bulky, adaptable, needs to be set up
Tent - 1.5kg (ish - slightly more i think) - bulkier, not adaptable, stands alone
Many variables to consider... you'd be really borlorxed if your atko ripped or the pole got damaged or something... however you could get by with half your tarp, or no tarp and just your bivi bag, if you had to. However the atko would almost certainly be far more comfortable if you were waiting out some seriously bad weather...
One thing I did like was that I didn't have any condensation...