Because the IBM offer is £10K more to spend far less time kicking tyres for a living. IBM do provide assurance to clients but it's a side line and not sold as a service offering in GBS. ITRA is still perceived as essentially an audit/assurance offering, and as such you'll find it very difficult to cash that experience in for a move into consulting later on (if you wanted to get into consulting, which as this is a consulting forum I assume you do). Neither is exactly going to set you up directly for a top tier MBA in due course, although frankly the options in IBM would be much closer to it. Yes it's big, yes it's blue, yes it's a corporate behemoth with all the flattening, dulling properties such huge organisations usually entail. But deep within IBM there are pockets of people doing some really quite staggering things. So the light has not been extinguished. Unlike ITRA where it never shone in the first place..