Sorry, you wanted pros and cons:
Electric steam steriliser:
Pros- Fits lots of bottles in so you can do them all at once. Items stay sterilised for (I think) about 3 hours if you keep the lid closed. Everywhere has a plug so if you need to take it away with you you can definitely use it, whereas you can't guarantee everywhere will have a microwave.
Cons- Bulky, especially when travelling, takes longer than microwave sterilising.
Microwave steam sterilising box:
Pros- Less bulky, better for travelling and if space on worktops etc is an issue. Takes less time.
Cons: Fits less bottles in, more chances of getting scalded from the steam/hot water as you move the unit from the microwave to the worktop, as electric sterilisers don't need to be moved.
Microwave sterilising bags:
Pros- Disposable after 20 uses so no cleaning, very very portable (they fold up to nothing) takes three minutes at full power. Good for when baby is screaming and you have no clean bottles, or when you only want to sterilise a single teat/dummy when you drop one on the floor etc
Cons- Only fits two bottles in (won't fit two avent bottles, rings, teats and lids in, I leave the lids off and just scrub them well) chance of scalding as before.