Deloitte gets my vote over PA, but some would argue that douglas is being a bit harsh on PA.
You might therefore want to look at the figures on this (FS revenues as a % of total revenues for both would help). You might then divide this by headcount to get their relative weights currently (although it would really be best to get their FS practise headcount figures, rather than using a ratioed weighting approach which might skew your figures disastrously). Unfortunately you might find that getting these figures, particularly figures on numbers employed (from their websites or from other sources) is difficult, and thus resort to other measures.
Non-quant measures including reputation can also be worth considering. People I have spoken to in the City about PA are not aware of them either, although somebody must have heard of them because they book a room at the Lloyds of London library building to show their city credentials to bright eyed and bushy tailed applicants. You might be aware that you recently missed one on Oct 3rd, although this is perhaps not all bad news. If you are an uncompromising optimist it indicates that they might well be looking to hire people.