From a comfort perspective, my preference for trains would override a marginal difference in journey time and additional 770yen cost. Train to KIX is a somewhat longer journey than bus to ITM, but any journey time advantage may prove irrelevant when you look at exact times of buses, trains and planes.

As an alternative, flying Jetstar direct from KIX to TPE seems likely to be a cheap and simple way to get from Osaka to Taipei.