I suppose if it was a long flight Id think about the upgrade. But a couple or 3 hours isnt that big of a deal to me.
An hour or two in conditions designed to be as uncomfortable as possible can make a short flight seem like a long, long one.
If your taking 2 bags, that's $25 for the first bag, $35 for the second. That's $60 each way. $120 round trip. There are no baggage fees in First.
Currently, r/t Coach, Chicago-Denver (2-1/2 hours), on United is $377 to as high as $1000 (most are in the $377-$550 range)
Same time frame, First class is $471 on up (most in the $471 -$750 range)
Often times it is almost a draw between the two.