How about both which is what I proposed?
Year 1: ICC World T20 (T20Is, 16 Teams, 4 groups, Super 8s - same format as the 2007 WC - 6 weeks)
Year 2: ICC World Test Championships (5 Day Tests, 8 Teams, 2 groups, Super 4s - 6 weeks)
Year 3: ICC World Cup (50 overs, 12 Teams, 2 groups, Super 6s - same format as the 1999 WC - 6 weeks)
Year 4: ICC Championships (3 match series in each format between the #1 and #2 sides - again max 6 weeks)
Everyone wins. Should also ensure that for the purposes of ICC Championships, which will be the league format for International cricket, every side should play the other 11 in every format in a 3 match series once over the 4 year period. Teams can schedule additional games/series outside of this but this guarantees everyone of the 12 33 T20I, 33 ODI and 33 Tests over a 4 year period. Which seems reasonable to me, and can be done inspite of the 12 weeks window we will need every year for the IPL + ICC events.