OK when you put it like that, it sounds odd doesn't it? It comes down to price v enjoyment; The e class is a much more expensive car and I had paid to upgrade my courtesy car to get the E, so I was judging a car that was costing me money to be in, to the insignia which was the free option. Much like in the real world, the E class costs more so expectations are different.
Price being not a factor, the E class is a better car of course. But at even £10 a day, I prefered the Insignia and here's why:
The E class was an AMG line with passive suspension (equiv. to the M Sport passive suspension I have in my G30) - handling wise it was all over the place; harsh over bumps but zero body control and wallowy during aggressive driving. It may have felt less wallowy on 20's like my G30, but that would certainly have made it even harsher on the bumps.
The engine was fine, but the programming of the ZF box was poor; it was often in the wrong gear and even in sport mode it didn't seem change gear any quicker, just changed up later. We know how good the ZF box can be so clearly this is down to how MB have programmed the software.
The infotainment system is dire and a complete pain to use - so unintuitive and laggy. The lag may be better with the upgraded full TFT widescreen dash. It felt like there was no consistency in the menus - you do ABC to achieve a result in the media menu, but to achieve the same result in the nav menu required XYZ. I was constantly frustrated with it, and I got on well with the systems in most of the other cars.
The MB sport seat is smaller than my G30 sport seat, and I'm a big guy at 6'6. There is also less leg room, which at my height, is a game changer.
Interior quality wasn't terrible but it wasn't great either - some bits felt cheap like the centre arm rest and the 'lid' over the cup holder in the centre console.
Lastly, I thought I would love the ambient lighting as there are more colour options than in the G30. However the way it is implemented did not impress me; it was not as 'sharp' as the line of light you get at the top of the trims in the G30. In the E, it was bright, but more diffused and less focused. I didn't like it, which was a big surprise to me.
Now the Insignia I liked because despite its more economical approach, I had lots of leg room, relatively comfy seating, and a basic but easy to use infotainment system that accomplished its few tasks without fuss. Nothing fancy but adequate without causing me any frustration. Also being a manual, I had no gripes with the gearbox. I love a good auto box, but I'll take a manual over a crap auto box anyday.
In summary, the E class to me was like a bad meal at a posh restaurant, whilst the Insignia was like a good meal at a cheap cafe. The first aimed high and failed IMHO whilst the second had a very simple mission and got those simple things right.