In the ITST v WT there is no comparison, in quality and certainly not credibility. WT generates an abundance of players because 90% of people who have the game will have a whirl of the preset Online Tour.
Of those that really enjoy topspin and / or are good at it will naturally hear or be interested in ITST by head hunts and recommendations. So whilst the quantity is envitably larger in WT the quality sits in ITST. I have literally come across no more than 3-5 players of perhaps 1000+ people I have played across TS3 & TS4 that had any real quality and sportsmanship that likened that of ITST ethos and standard; in all of those some 3-5 players they had all heard of ITST but did not play largely due to playing restrictions, work etc.
I can also vouch more recently even in TS4 WT that any so called "top WT" players were again very average, Polakis for one rings a bell. Many a time have WT hall of famers come to ITST and sat outside the top 20. Bottom line is, ITST rules and it is a shame that the active members has not increased in the ladder for TS4, actually decreased in my eyes, but I am sure a solid 5th series in topspin series will sort that out.