I ride the maglev regularly at different times of day and have to say i don't notice much difference at the different speeds. The ride is so short anyway and it will only max out for a minute or so on the straight stretch.
The only reason i can think of for the slower speed at night is the burden to the residential areas it passes through. Though i can't imagine there is much of a difference whether it's 300 or 440 kph. Could also be there are less passengers taking it at night so no reason to have the trains running faster, thus more often.

If i were in your shoes and it'd be the first and only time to ride the maglev i'd just wait the extra few minutes, grab a coffee and then ride the fast version. And i'd catch a cab from the main road and not from the cab line at the Longyang station, less chance of getting screwed by the driver. Shangri-la hotel should be a 20 minute ride, maybe more since you're there in the morning rush hour. Pay no more than 30-40 RMB.