Pakistan team is very unpredictable they can lose an easy match and win an almost impossible match. Pakistan's bowling is always the best, they produce the world's best fast bowlers. Sri Lanka will be high on confidence in this match as they have bounced back from a very bad first match against Afghanistan. In this match too, the toss may play a big role in the match result.
Which team isn't unpredictable in the T-20 format to be fair? It's just the nature of the format itself. Considering everything, Sri Lanka seems to have a more balanced side when compared to Pakistan.
This is why I feel that betting on Sri Lanka even though they are the underdogs is the right call. Also, the higher odds are another reason.