What is your total allotted study time if you were to take the written? 3 weeks longer may not be as beneficial as you might think if you already have 5+ weeks to study. General concensus around here seems to be that 3-4 weeks of SOLID studying (8-12 hours/day) is ample time to do very well on the exam.
On the flip side, while the computerized does cost $100 more, you would get your score back 2x as quickly as with the written. Also, where are your nearest Sylvan testing centers? For example, I was VERY excited about the computerized exam until I discovered that the closest testing center was 25 miles away. No big deal, except for the fact that it *could* take me 2-2.5 hours to get there during morning traffic.