While counting heads up is better because you will see more hands per hour. While counting time really is money. You want to get as much out there as you can while in a high count. When you have every single hand to yourself at a high count thats much much more ev. also the mroe hands per hour the faster you will reach the long run which is the ultimate goal.
When playing at a full table it is best to be at the last seat. this way you will see all the action before you giving you that much more information to influence your final decision.