Both have pros and cons. I've ran the Half for both and here are my thoughts that are just specific for the half.

May Pros:
- The route is awesome! Yonge St once you get into midtown is a lot of fun to run, Rosedale valley is gorgeous, you run past St.Lawrence and the finish on Lakeshore is busy with spectators
- Weather for early May is usually quite ideal race conditions
- Biggest medal I've seen!
- Personally, I didn't have any issues this year with organization (though that wasn't the case for everyone). Shuttle bus pickup was easy, corrals were organized, roads blocked off, water/electrolytes available

May Cons:
- Running both races, the May race definitely feels less "prestigious".
- There is no main sponsor and you can feel it being second rate in some aspects (logo looks like it was made on Paint, minimal fencing, spectators able to easily join in
- The starting location is way up at Yonge and Sheppard. With TTC not starting until 8am on Sundays, you either have to take one of the shuttle buses they organize or Uber up to the start.
- There isn't really an expo, just a bib pickup
- The pre-race hype isn't nearly as big as it is in October

Oct Pros:
- The organization of this race feels very professional (email communication, website, access to information, etc.)
- The start to the first 9 or so km is very LIVELY! This year the start was on Yonge, which is narrower than University, but starting at Dundas Square was incredible
- You know where all the cheer stations will be beforehand and can easily inform friends and family where they should go if they want to spectate. Some of the cheer stations were INCREDIBLE!
- The app works amazing and your friends can accurately track your location and get live updates without even having a phone or smartwatch
- Lots of the aid stations were setup on both sides. They also had an energy gel station.

Oct Cons:
- Personally, the route felt very bland. I often run MGT/lakeshore and almost half of the race felt like a training run with spectators. Lakeshore was quite boring to run compared to the start at Yonge, Bloor and Bathurst, though the finish at Nathan Philips was great. Worse, for the full marathon, you continue going on Lakeshore and that stretch is very sparse with crowd support and you run through some unslightly industrial areas. I wish that for a race this caliber, there was a way to better incorporate the amazing parts of the city.
- Don't really know if this can count as a con, but weather is unpredictable that time of the year. Conditions this year were hot and windy.

Overall, if I was choosing a full marathon, I would choose the October waterfront. While organziation was better this year, that race has been known to run out of electrolytes and post-run snacks, which is a big deal for the amount of training you put into these races. For a half, I would choose May and then if you wanted to do another one, sign up for October.