Boots has the high end brands and some brands exclusive to Boots. However, basic items are way more expensive than Superdrug.
Superdrug has few (none?) high end brands and limited range of perfumes. However, it does have GOSH and Sleek as well as a few other brands only available in Superdrug. It's way cheaper, NUS 10% off and the loyalty card is better - can pay part of bill in points which you can't do in Boots. I find the Boots advantage card inflexible and given the extra cost of most items, really isn't much of an incentive to spend in Boots.
I try to buy in Superdrug and whatever Superdrug doesn't have I'll buy elsewhere, maybe Boots but I rarely buy high end makeup in Boots. More likely to buy from department store or online. If I'm going to buy some expensive ****, I'd rather do it somewhere fancy or with a bigger discount.
There are some things I will only buy from Boots, not Superdrug e.g. cotton face pads. Quality of Superdrug's were so poor. Some of Superdrug's own brand items are of questionable quality, whereas Boots are usually ok - decent.