E36 M42's also got some nice upgrades over the E30 M42's...
Better intake manifold design with the DISA valve, Better designed oilpan, (after '93) better profile gaskets, and if I remember correctly, they got stronger valves too.
I'd still rock a 91 318is though, no question about it!
Yes, E30's are definitely lighter, but honestly, my E36 doesn't feel piggish at all. I can toss it around like crazy.
Yes, E36's are definitely stiffer. My E36 is just so tight feeling and amazing to drive. My E30 is also wonderful, and is just a fun car to take out and ride in. The E36 definitely makes a better daily driver, seeing as it's a little bigger and more comfortable (depending).
I can honestly say that I have become a more competent driver not only because of continual experience, but having to cope with such an "underpowered" car makes you really drive differently. I have learned how to tune into a car quite well, and when I am in my E36 it becomes like another appendage. I literally feel everything through my seat, steering wheel, and pedals. I know what my car is doing all the time. It is a wonderful feeling.
It's the feeling of an M42 powered BMW!

Cliff Notes: My advice? Get what you really want. Either way, you cannot go wrong! E30's and E36's are awesome for different reasons.