My notes show 329 as "mostly full", 331 as "50%", and 333 as "25%".
Option 1: Can you leave 2 hours later on 333? Option 2: If you absolutely must have a window seat (as I do), you could take Metra scoot 2112 leaving Glenview at 7:19am and arrive at Chicago Union Station at 7:54am. That should give you enough time to position yourself at the front of the crowd and take your pick of seats when 331 loading starts. That would make you the Amtrak equivalent of airline "gate lice", but hey, window seats are important.
I agree with Grandpa D. Neither side is very interesting, but the east side will give you a brief glimpse of Lake Michigan just before arriving in Milwaukee. It's pretty when it's sunny. Also, the east side will get you a look at Amtrak's Milwaukee Airport station. It's a real gem of Prairie Style architecture.