I have a Bosch 55cm cordless hedge trimmer. I prefer the cordless models, because I have some hedge quite a long distance from the house, and, in any case I don't like the trouble of messing around untangling cables, and also hedge trimmers and electric cables don't mix well, IMO. Though they should be safe if operated with the recommended cutouts.
The hedgecutter that I have has a long blade, and is powerful, at least for my requirements. Bosch suppy the trimmer with two batteries, so you can swop batteries at half-time.
I have some other Bosch tools - a cordless strimmer and a shreder. They are all well-designed and I have never had any trouble with any of them. My only complaint is that the hedge trimmer batteries are not interchangeable with the strimmer battery.
The only difference between the two models you were refering to is the length of the blade, They are otherwise identical. In general, if you have a lot of hedge, then larger models are a big advantage, because the blade cuts a larger area, and therefore gets the job done quicker. But for a small hedge around a small/medium garden, a bigger model is not an advantage.
The prices of those two models at Amazon are:
Bosch 45-16 (45cm blade), £45.95 + carriage
Bosch 55-16 (55cm blade), £62.99 + free carriage