Even if you have good chemistry with your preferred MPA, for myself an hour is rarely worth it. Personally I find very few girls have enough moves, variety and energy to maintain a high level session for an entire hour.
When you book forty minutes that's not supposed to included your starting or finishing showers (technically the time starts ticking whenever the MPA first enters the room, and in most cases you're already done your initial shower by that time). If you skip the massage altogether then 40 minutes is actually fair bit of time for a lot of fun and some pillow talk after.
Even though you only get an extra twenty minutes when you bump up from forty minutes to an hour, the extra expense is at Premier is $100, which is a lot given that the first 40 minutes is only $150 (before any extra expenses of course).
If that extra 20 minutes is jam packed with high level service that's certainly an argument to opt for an hour, but if she's running out of steam and ideas and you're not really getting much quality time or even your full paid time during the last twenty minutes you're paying an extra $100 for, then that's a good reason to stick with forty minutes, and save your extra $100 for your next 40 minute appointment with someone else...