For Grandma’s I decided to wear my 305 on my right wrist and my 405 on the left. I started each one just as crossed the start line. According to Training Center and Ascent (log program for the Mac), the 305 was started 2 seconds after the 405. 7:35:21 AM and 7:35:19 AM respectively.
I did not do mile splits but did do a split at the half. The 305 was 13.1 in 2:46:18 and the 405 had a 13.1 mile split of 2:44:56 (2 seconds off reported actual race time), What is really interesting is that both watches had my finishing time within 2 seconds, the same as the start, the 305 on the right wrist had 26.37 miles while the 405 had 26.22 miles. This is a difference of 0.15 miles.
The 405 had a total ascent of 3,285 feet and a decent of 3,395 feet.
The 305 had a total ascent of 3,056 feet and a decent of 3,162 feet.
Average and Max/Min heart rates were the same on both watches.
Therefore…
I could assume the course was longer on the right side a runner yet more hills on the left?
I could assume that the 405 is more accurate than the older 305?
I could assume that the reception on the left was better than the right?
Although, I will just assume it is science, and science will never be perfect… No matter how much we try or expect perfection.
Carry on…
Tryingtorun's Blog 
Just saw your comment elsewhere about the ACL and meniscus tear. Heal quickly, my friend. It's amazing that you finished Grandma's on that knee.
At least the course wasn't too much longer than 26.2. I think one of the marathons I ran was .7 longer. Ugh! I'm actually surprised that the watches didn't come out to the same distance.
The 405 antenna is advertised as being better than the 3xx, but GPS elevation is NOTORIOUSLY bad and I think it generally exagerrates high. Back in the http://www.motionbased.com days (before it was bought out and became Garmin Connect), one of the options they had was "elevation correction" where they traced your route and based elevation on topo data. Not sure if GC does that or not.