I voted Always on.
However, depending on what networks are actually available I will sometimes be in the 'Aways available' category instead, in other words: I prefer to be in control _sometimes_. On other networks I instead just stay 'Always on'.

As for the battery drain, I've been looking at this a bit. I experience big differences depending on how I'm connected. At work I'm online the whole day (8-10 hours), with almost no visible drain to the battery at all: At the end of the day I'm still at 9days/6 hours. But at home I have a somewhat flaky connection which sometimes is very slow (the wi-fi router -> internet connection even has some packet loss now and then which slows it down even more), and here the N800 battery drain is dramatically higher.

I've previously mentioned that I have another router which always drain the N800 quickly, this seems to be the router itself which forces the N800 to never go into wi-fi power savings mode.

So, to stay always-on with low battery drain:
1) A high-speed, low-latency internet connection is best (presumably because it lets the Internet Tablet use wi-fi in quick short bursts, which lets it go into power saving mode more quickly).
2) Beware of any other issues (e.g. router problem) that could prevent the Internet Tablet from going into wi-fi power saving mode.
3) Online via BT doesn't seem to drain the N800 much, but unfortunately my phone definitely feels the strain after a couple of hours. So, get a good phone..