I can only give you my opinion on the one we bought - the Philips Avent Natural Microwave Steam Steriliser. Like you, we fully planned to bf but right from the start had to top-up with formula and at 2 months are exclusively formula feeding, so this steriliser was a god-send from day one.
The steriliser itself is amazing, but be careful whichever one you buy, as it comes with specific bottles. So for example, the one we bought came with bottles that had 'natural' teats instead of classic, which are more like the nipple apparently. Unfortunately we didn't realise there were different types of bottles, so later bought Avent classic bottles, which have completely different teats, and thus confused and frustrated the hell out of our little one when she was suddenly using different teats.
This steriliser is £26 from Mothercare, you pop 200ml of water in it, it can hold 3 bottles (broken down into all four pieces) as well as 2 dummies, shove it in the microwave for 6 minutes and done. Simple as anything. Obviously it does require a microwave to use though. Also make sure to get a bottle brush, and always rinse the bottle parts in warm water before sterilising.
Best of luck with whichever one you choose :)