I wouldn't worry too much about energy usage differences; the main way a washer saves energy is by having less water to heat. Front-loaders already use far less water than top-loaders, so saving those last few liters isn't particularly useful unless you're reliant on tank water.
The thing is, unless your washer is exceptionally well designed (eg Asko) and correspondingly expensive, too little water translates into insufficient rinsing.
I am not making this up.
What does matter with front-loaders is the length of their cycle and how well they cope with out-of-balance loads. Many take well over two hours to complete a cycle (it's how they wash so well) - more as your inlet water gets colder. That's hugely inconvenient if you need to quickly wash a single garment. I strongly recommend downloading the user manual to get these details and find out how you would actually use it, eg. is there an optional 30-minute cycle?
The ability to deal with unbalanced loads is hard to determine before purchase, but more expensive brands tend to be better, and they may make explicit claims about it. Otherwise, keep a couple of towels handy to balance problematic loads ...