I haven't tested it specifically with the Touchpad but, on my Pre using
As Items Arrive always used a lot more juice. Many people mistakenly believed
otherwise, but never bothered to properly test it. Email servers are fundamentally
about polling (they have no push mechanism), so the only way for a remote device
to detect incoming messages on the fly is by keeping a connection open to the
server continuously which means the radio is always on, and that uses more power.
I regularly get 48-60 hours standby on my original Pre- (original battery)
with 30 minute polling with 3 active email accounts and significantly
less, using As Items Arrive. Given that experience, I've no reason to
change that on the Touchpad.