Hmmmmmmmm ... that's a tough decision. I hate sitting around airports almost as much as I hate sitting in traffic. But, I think I would take the earlier flight just because LGA (and all the NY airports) can be nightmares if there's bad weather -- even just rain. I wish I had $1 for every extra minute I've been delayed on flights involving EWR, JFK and LGA!!
About this time last year, I had a flight from JFK to Athens. Weather channel was predicting heavy snow in NY. I had bought two separate tickets -- one to get me to JFK ... and then the international ticket (I wasn't sure when I was returning from Europe, wanted to avoid as many change fees as possible and was doing things somewhat peacemeal). I called Delta the day before my flights and asked them if I could take the domestic flight a day early (same day) based on the weather predictions -- explained I needed to get there to make my international connection (also on Delta). Surprisingly, they let me -- with no extra costs. I ended up just spending the night in Manhattan, having dinner with friends -- it worked out beautifully -- and many of the next day's flights were cancelled so there's a decent chance I wouldn't have been able to get to NY to catch the Athens flight if I didn't go a day ahead of time. And, while most of the domestic flights were being cancelled at JFK the day of the snow storm, they got the vast majority of international flights out. A friend that I was meeting at JFK did the same thing ... took the redeye the night before and got there at the crack of dawn for a late afternoon/early evening flight ... bought a one-day pass to the President's Club (or whatever Delta calls it), showered, napped, etc! Bottom line: If we hadn't gone early, we would have not made our Greece flight.
So ..... I would say take the earlier flight ... not because of traffic between LGA and JFK .... but due to potential delays into LGA. And, if it's really early, just go into NY a day early and enjoy yourself! It breaks up the long trip and removes the stress!
And ... to answer your question ... yes, there can be HUGE snarlups getting that very short distance from Jamaica to JFK. Millions (?? probably an exxageration but LOTS) of people need to take that highway to get home to Long Island each evening. The whole area is a giant traffic nightmare.