Posted a similar query yesterday, which was moved to the thread https://www.flyertalk.com/forum/unit...ht-2018-a.html

No immediate upgrades available on the routes my husband is traveling (SFO-PEK, HKG-SFO); was considering booking M class, which cost $1K more than the lowest upgradable coach fare (W).

The advice I got (from jsloan - don't know how to do a quote from one thread to another) was:
"...the first criterion is instrument type (GS GPUs trump 1K GPUs), the second is status, and the third is fare class. Most 1Ks with a waitlisted GPU will be in W class. There could be an argument made to buy V class in order to skip ahead of those passengers. There's no argument whatsoever for M class if your goal is simply to improve your upgrade chances. You're just throwing your money away.
There's a pretty reasonable chance that there will be zero passengers with 1K status with a waitlisted instrument between fare classes V and M. Even if there are, the money is only a good investment if you clear in M but would not have cleared in W/V. So, it's a very specific set of circumstances -- there have to be enough seats for you but not enough for everyone. If everybody clears, you've wasted your money.
My best advice to to find something with open R space at booking. However, it sounds like you don't have a ton of date and route flexibility. In that case, I would either purchase a W fare (or V, if you wish) and waitlist a GPU, or the lowest possible fare and waitlist a copay + miles upgrade. (It depends on the spread between the lowest fare and W/V)."

Went with V class (cost about $60 more than W). Also helps to get on waitlist as early as possible. We are about 2 1/2 months out, and there are still lots of unsold business class seats. We'll see...