I'd be willing to place a straight wager on the taxi in this case, in light of the following:

-minimal wait time for taxis at 230
-not much traffic, especially between the airport and Longyang Rd.
-in light of his baggage situation, the OP would spend at least 10 additional minutes (i.e. assuming boarding taxi = minute zero) simply walking to the Maglev station, purchasing tickets, and getting on a train
-then toss in the 10 minute average wait time (ranges between zero and 20 during peak schedule)
-at the Longyang Rd. part of the journey, tack on another 5 minutes to disembark and fetch taxi
-add another 3 minutes for the Longyang Rd. taxi to get to the road that leads to the bridge

--> all told, that's 28 minutes of idle time. Add in the 8 minute Maglev ride and you're already at 36 minutes.

From that point on, the Maglev OP and the taxi OP are on even footing (i.e. both are in taxis headed towards People's Square). However, taxi OP probably crossed the river at around the 32 minute mark, so he should win by a hair.

Naturally, if Maglev OP gets lucky on the Maglev schedule front, he would probably prevail..... but, based on my calculus, taxi OP has a 70% chance of winning.

But, as you intimated, the nominal time differential isn't so important in a city like SH (i.e. I've been in taxis that needed 40 minutes to travel two miles). Comfort is king.

Closing note: geilux: you should still try the Maglev at some point in your life. As an FTer, you would enjoy it for the sake of its very existence. But, if your companions are anything like my girlfriend, they will be less impressed.