My first BMW was a 316i Compact back in 1997; I drove a 318iS about 4 years ago & was impressed; the 316 always felt underpowered; the 16v mill in the iS/ti certainly makes a huge difference.
I would guess that the insurance difference is to do with the price & placement of the 2 cars back in the mid/late 1990's - the Compact was a lot cheaper than the coupe as it used a lot of surplus E30 parts (the older 3 series) such as rear suspension & switchgear. The Compact body was related to the saloon more than the coupe despite being a 3 door. The coupe was lower, sleeker, more modern (despite being released 2 years earlier than the Compact), better engined, generally faster and more appealing than the Compact.
When I bought my 316i Compact in 1997 I paid DM30,000 for it (£10k) - it was 6 months old. The equivalent coupe at the time (a lowly 316i with cloth seats on the same forecourt (BMW Niederlassung Essen)) was DM40,000 - 33% more.
I loved my Compact though, it was so well made, economical, quick enough & had brilliant brakes compared to my old Peugeot 309GTI!!!