Peugeots generally have rather poor reputations for reliability, although, to be completely fair an amount of this will be squeaks, rattles and general lack of quality rather than big expense items. But that's enough to hit their desirability as a used purchase.
The Insignia is almost certainly a lot better than previous generations of Vauxhalls. It would be less of a 'driver's car' than a Mondeo, but if you are more into the space/comfort than driving pleasure, it would still be quite a good option. Reliability is probably inferior, even though both are 'mostly reliable with occasional forays into significant expense' cars, which is a particular issue with diesels.
Also, while the Insignias (diesels) claim really good economy figures, you'll find the internet is full of people claiming that they miss the official figures by very wide margins. While all of the official figures get missed somewhat, this is really quite an extreme effect, and it is a bit unclear whether Vauxhall have just found more dramatic ways of 'tricking' the official testing, and the cars are ok-ish but the official figures are illusory (over-promising and under-delivering), or whether there is something more serious going on, like some systematic faults.
Where are you looking to see a range of vehicles? For this, I find Auto Trader is the best web site, but Ex & Mart and Motors.co.uk are also useful, the latter if you want to set a spec and see what cars are available, irrespective of model.