Timewise it's not too bad if you leave early enough. I was commuting to Tarrytown from Beacon and as long as I left by 7:30 a.m. I was able to get in by 9:00 a.m. along both of those routes. Customarily I would check traffic reports (which you can now do online at maps.google.com - click on Traffic in the upper right-hand corner of the map. Colors will indicate various conditions: If available in your area, real-time traffic conditions will be displayed over the highway as color coded lines. Each color represents how fast the traffic is moving:
Green: more than 50 miles per hour
Yellow: 25 - 50 miles per hour
Red: less than 25 miles per hour
Gray: no data currently available)
Then, depending on the results, I would decide whether to take 84 to the Taconic or whether to go take 9D to 9 to 9A . I found the Taconic to be fast and winding with an overabundance of agressive drivers (I think I was one of them :-), and Route 9 to move more slowly, and which seemed to take forever. Traffic on the Taconic would predictably slow down around Yorktown and as you approached the Saw Mill River Parkway and beyond. Traffic on 9 slowed at Peekskill and closer to Tarrytown. It was an excruciating commute for me. I now live in Ulster County, about the same distance, and the commute using the NYS Thruway is much faster, easier and (IMHO) safer. Hope this helps!