If I book in M class am I more likely to get an eupgrade than if I book V or S class?

It's for the YYZ-LHR and LHR-YYZ legs, in September. I'm a lowly 50k'er.

Yes, you are more likely, however nothing is guaranteed.
The cost of an M fare is very high, might be better to book Z if available.

I just checked a few random dates in september and M-Fare is 2xFlex. (1100 each way). It will be easier to get the upgrade, but no guarantee. How pissed would you be if you spend the extra dollars and the upgrade didnt clear? I am E50 and had no issues to YEG from LHR in July

Why would OP be more likely?

Aren't both wait lists (gate waitlist and pre-gate wait list) rank listed by time of request? So SE trumps E75 trumps E50, etc.

If not, nobody knows the upgrade algorithm, so it is possible that OP could pay the M fare premium and still lose the upgrade lottery.

Yes, but still, try to book Z if it's not that much more. Also, I'm not sure how many daily flights there are out of YUL, but you may be better off doing YYZ-YUL-LHR, because connecting pax get priority that may even trump fare class (though no one knows for sure). Of course, you're probably better off on a 777 from YYZ, than on a 767 from YUL, just because there are so many more seats on 777. But if you can find a 777 from YUL, then with M class + connection = great shot at upgrade.

If he is E50 on M, he would have better change then any E50 on any other Flex class.

LHR & SYD are the 2 of my business Travel Destinations.

If you are traveling on a weekday, YOW-LHR even with 767 odds are very high getting an upgrade.

EX: YUL only if it is 333. YUL for years was 333. Last year AC changed it to 767.

From YYZ I only book flights on 777 or 330. Even as SE I would not risk 767 from YYZ.

Ok but in the whole scheme of things:

Connecting passengers (?)
SE100 on Y/B/M/U/H/Q/V/W/S
E75 on Y/B/M/U/H/Q/V/W/S
E50 on Y/B
E50 on M (OP)
E50 on U/H/Q/V/W/S

OP is near the back of the list.

Should OP start the trip from YTZ (if AC allow) or YXU instead of YYZ? That will put OP at the top of the list.

Or like princess said; connect in YOW;

Although I do not regulary fly LHR/FRA from YOW, I know my research has shown they are easy to, upgrade, and as an SE I would say I get ~30% OP-UP upgrades for flying out of YOW because they do have a tendancy to oversell Y here and J very rarley is full.

I second the suggestions to add a connection. If your schedule permits, it's better IMO than spending the extra $$$ for an M fare with regards to chance of upgrade.

We don't actually know that for sure.

Again, we don't actually know that

I feel like I'm repeating myself

However, when does the summer upgrade promotion end? If the upgrade can be requested within that period, M fares have a 14 day window, which can trump all status.

But as was stated earlier, M fare is often fairly close to Z. I know which one I'd pick

Edit: I should add that I've had IRROPS clearly put me near the top of the list, when I was previously waitlisted, but I've also had IRROPS move me from a confirmed upgrade to near the bottom of the list. So that's why I'm so skeptical of our guesses about priority.

I have read often that connecting pax have upgrade priority. Is there empirical evidence to justify it?

Concierge at YVR told me so. This may be a bit different because it was YVR - SYD (same flight number), but he told me an E35 (this was a year ago) boarding in Toronto with a Tango Plus and who upgrades from YYZ to YVR, and gets wait listed for YVR-SYD, TRUMPS SE ON LATITUDE, going from YVR to SYD!!!!!!!!!!!!!! He told me it was outrageous and that I should write in to complain. Again though, the same flight #, might have something to do with it.

Additional evidence, though not of a very compelling nature, is a non-status PAX I upgraded last year YYC-YVR-SYD, who got upgraded at the gate on YVR-SYD, though that might has just been a fluke; and the fact that in the past two years, I've only missed one upgrade YYZ - PHX, and upgraded dozens of others as a connecting PAX.

Actually, also, I distinctly remember, trumping four other SEs for the last YYZ-YQR seat, last year, and I was only flying on a tango plus ticket, and had very year to date miles at the time (and the year before I barely qualified for SE). Unfortunately I don't know what fare class their tickets were, but I'm guessing the winning factor is that I had just flown in from LAX, and that obviously got me the seat. Although this this part is pure speculation, they were all wearing suits and travelling for work, and so I wouldn't be surprised if at least one of them had a flight pass (i.e. M class).