What is the difference between the Micro Terror head and the Micro Dark?

The main difference is in the level of gain available. The Micro Dark is significantly more 'Metal' sounding, with higher levels of gain and a tweaked EQ that makes the amp sound more aggressive. The Micro Terror still has punchy gain but is more suitable to Classic Rock, Blues and Indie playing. Both amps sound fantastic and are well worth further investigation!

I'm looking for a versatile amp head that will give me a great high gain sound as well as a great clean sound. What do you recommend from Orange?

We'd go for the Rockerverb 100 MKIII head. It's a serious investment but there are few high gain sounds out there to match it and the extra headroom provided by the 100 watt power (the Rockerverb is available in a 50w model too) allows for pristine, chiming clean sounds. It is an incredibly versatile head.