Boris K
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I left the purchased former enemy heroes to defend. I simply didn't have enough money for troops, and there were serious resource problems (primarily with ore), so I couldn't build many dwellings, which meant there weren't many creatures in the cities. But I had plenty of experience. I completely ignored upgrading Shadia – she was just a loyal retainer, and I had Railag visit upgrade locations (there aren't many of them, if any at all – I don't remember exactly) and built intrigue chambers everywhere. As a result, in the last Campaign, Railag was the most powerful hero. Although, in general, I prefer Warlocks, and it's easiest for me to play as them.
In my opinion, the AI upgrades its hero in each city. Therefore, having two cities, you can assemble an army for your main hero that is almost twice as large as any enemy army.
I'm not sure if it's easier to play as Warlocks than as others (I don't know Druids and Mages at all, since I only play the Campaign), but, in my opinion, it's more interesting. They have so many good development options. It seems easier to play as Humans and Necromancers in 1.41. Hordes of crossbowmen and skeletons sweep away everyone.