Hi,
Depends on budget, how often you will cut the grass, & your time to look after the grass... I'd go for a petrol one myself.. self propelling, adjustable cut height.
I've a large area aswell & re-seeded one large section in 2009... that side is great now... was able to cut with a push mower last summer, as grass was short (was able to cut it often, twice a week to keep it at that height) & it's not used too often. Other side of garden, with influx of weed / moss & less enthusiasm to re-seed, then the petrol one comes in to get it under control & keep it under control.
To keep the grass in shape, you will need a strong motor, sharp blades & adjustable heights - not too sure if there are any electric lawnmowers out there that can give you that as good as a pertol one.
Also think of the time that you may miss a cut due to bad weather / holidays... you don't want to be struggling to get it under control again...