I've done both.
The train trip....the ride down is amazing, everyone is getting more and more wound up with every road crossing and as kilometer of track takes us closer to the game.
Coming home, everyone is still in good spirits but the last 2 hours are tiring and be ready to sleep sitting up.
That being said a great trip all around, I wouldn't take kids.
I found flights to Montreal on Westjet last summer for ...wait for it....$12 down and $27 return and then found a great deal at a hotel in Old Montreal.
While more costly, seeing Old Montreal in the summer is something that shouldn't be missed. The food is amazing the nightlife is second to none and the sights are awesome.
The down is, the Ticats only get enough tickets for the train takers. So I had to contact the Al's on my own. We got tickets in the same end zone across a large gap in the stands. It also is more expensive then the train...but worth it.
I would do both again...as both are fun.
Hope this helps.