Spec wise, the 98 is 11.3 and the 106 is 10.9 (strung). Disregard the TW specs, they are a bit off. Length is 27 for the 98 and 27.3 for the 106. Both are soft flexible racquets. The 106 is 22mm wide with a 18x19 string pattern, while the 98 is 21mm wide with a 18x19 pattern. Most of the pros use the 98while there is some debate whether Serena and Venus use the 106.
The swingweight and shape of the head is the biggest difference initially, then control. The 106 swings lighter even though it is a bit longer. More pop but less control. I thought the tighter pattern (18x19) would help a bit but the ball can fly on you (the 106 has a generous sweet spot, this racquet is great for doubles). The 98 has more control and less pop even with the greater swingweight. Both racquets have great dwell time from the soft flex, but watch out when hitting the ball on the frame with the 98, it will go nowhere. Serves go to the 98 as it hits high in the hoop. Volleys go to the 106. Both racquets are great for customization. Feel is the big reason why all players love these racquets, very soft. And unfortunately, both racquets feel differently. The 98 is a more traditional feeling tip high racquet, a players racquet in a lighter weight. The 106 pretends to be a players racquet but in the end, not quite but still good (it is definitely better than most tweener racquets). I like teaching with the 106, it is fun. Hope this helps......