ok, I'm contrarian to most of the previous posters. I'd pick the Scottish Highlands (although you do realise thats a considerably larger area than the Lakes, with a lot more variation? but more of that presently) over the Lakes. I find the Lakes scenery pretty mundane, and the excessive number of people trying to walk there disappointing. Towns are too touristy, some of them verge on tacky. By contrast, Scottish scenery (depending where you go) tends more to the dramatic and the magnificent than the vaguely pretty but over populated, and with a lot fewer people around - you don't generally have to queue at stiles like you do on some of the popular Lakeland walks, and path erosion is not such a big issue.

Weatherwise, the eastern Cairngorms are considerably drier than the Lakes, although the Scottish west coast is prob wetter than both - although there might not be so much in it between the Lakes and the west coast. However, you do have to be more weather aware in Scotland, as a lot of the high uplands are closer to arctic than alpine in climate, so weather changes can be swift and nasty - but this is more true in winter than in full summer, but still something not to forget. But in summer, its a matter of always taking waterproofs, extra clothes, maps and emergency chocolate... not really a big deal, because if you're sensible and experienced, I'd guess you'd do that as a matter of course.

Okay, what about difficulty. Yes, parts of the Highlands are higher than the Lakes. But its such a big area, you don't necessarily need to go there. And to be honest, even getting up onto the Cairngorm Plateau is at most only going to be a few hundred feet of your vertical limitations, because of the starting altitude - the highest mountain in the UK is a bit over 4000 ft, and you don't start off at sea level Still would consider it more than modestly strenuous though If I were you, I'd be tempted to base somewhere like Dunkeld, Pitlochry, Aberfeldy or the Aviemore area (not Aviemore itself, its a concrete hole), where there is a range of walking available from short woodland walks to longer harder stretches. All of those are eastern Cairngorms, which are drier, but with great scenery. They can get quite touristy as well, but still will be quieter than the Lakes at that time of year.

As for touring - I've visited both independently, so I've never needed the tours, so I'm afraid I can't help. One thing I would check out is what exactly do they mean by "highlands" and how long do you spend in a bus versus actually walking? As I say, the Highlands are a pretty big area.... much bigger than the Lakes, and it will be key where they go. Shout if you need any specific information on places they go.

Oh, and wherever you go, bring the midge repellent