Anyway your use of averages in this way is highly debateable IMO. A spinner of equivalent value to a pace bowler will generally have, as a fair rule of thumb, an average of 4-5 runs greater. The bench-mark for an alltime great pace bowler is probably an average of 20-22, a spinner more like 24-26 (compare McGrath and Warne for example - who is the greater?) A merely "good" pace bowler can probably be considered anything under 30, and similarly the benchmark for "good" spin bowler is probably 34-5.
The reasons for this are probably multi-fold, including such things as spinners doing a lot of the donkey work when the pitch is flat and the ball old, the fact that they only generally come into their own as wicket takers in the second half of matches etc etc. It is also much less common for spin bowlers to boost their average with cheap wickets. There is real value in these things which won't be reflected in the averages which is why they can always find a place in a test team despite statistically inferior figures (as long as they can bowl economically - so for example the case of somebody like MacGill is more contentious), both for the role that they play in improving (through resting) the figures of the pace bowlers and the invaluable role they play in winning matches when the crunch comes in the second innings.
In 2005 Giles' role was solely to do the "donkey work", but so good was England's pace attack that he was almost worth his role for that alone. However on the one occasion when he was called upon to win the match in the second innings (Old Trafford) he was found wanting. Such were/are his limitations. He needed to make some contribution with the bat to compensate.
Monty on the other hand has demonstrated in his short career (and yes, it may be a false dawn, we don't know) has shown he is potentially on a different level. He has more than demonstrated that he can do the first innings donkey work (and far more effectively) and also that he can win matches in the second. Not always (Lords vs Pakistan) but he has done it (Old Trafford, Headingley). If England had their first choice bowling attack (inc. Jones) AND it was firing the Giles/Panesar call could still be potentially very close. But our pace attack is a shambles, so really it's a nobrainer.