I covered MMW (Magic Market Week) or "Fashion" week in August 2013 for a local Vegas magazine and I noticed the traffic here is getting worst on the daily.
With that experience in mind, I think Bally's would be best. At least, for convenience. However, there are much visually nicer accommodations in the same block of the strip. For instance... Paris, if I'm correct, is right next door and even connects to Bally's or Bellagio just across the street, which has a sky bridge that connects as well. Both Paris & Bellagio are higher end facilities than Bally's, but also tend to cater to an older, mentally mature, and subdued crowd.
I say "for convenience", because Bally's has a Monorail station in the building. Which, also stops at the convention center. Then you would only have to contend with the strip traffic between Bally's and Mandalay Bay, and if you know your way around a couple back streets (Koval in particular) you can pretty much avoid driving on the strip all together.
Just for reference & so you know, that I know what I'm talking about...
Here's an old hub of mine: "A First-Timer's Guide to Visiting Vegas" https://hubpages.com/travel/VegasVirginsLookHere...
Hope that was helpful. ~ Good luck!
Kevin Marsh - www.facebook.com/klfm3