Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Unfortunately, you only get to pick one.
The fastest route will likely be the most expensive (probably $100 or more for a taxi or car service with tolls and tip); the cheapest route (~ $5.50) will take about two to two and one-half hours, and require two buses and two trains.
In between those two extremes, you could take one express bus from LGA to Manhattan, and another from Manhattan to EWR. That would cost about $25 +/-, and take about 1 1/2 to 2 hours. Another option is
http://www.etsairportshuttle.com, which
might be a one-seat ride.