I can't comment on Online and I'm not running it down; I only use SE.
Uptime for my wife's SE system was 1 year, 10 months. Then the Sun died :(.
Reinstallation from backup to the other Sun, 45 minutes. Back in business. I
know of other SE databases where the uptime is in years; my genetics one, for
example. While it's low intensity, my wife's system logs data on over 100,000
patients per year.
SE is stable, reliable and fast enough for what I need to do. Highly recommend
it.
Peter Wiley