I would personally say the two are too different to compare them directly as the style and sound of IM changed quite a lot when Bruce joined them. Bruce is definitely a better singer by far, a great front man who can command the crowd and get them involved in the performance. The only man who could beat him in that respect has to be the great Freddie Mercury. But DiAnno managed to generate more raw and original energy in his vocals and IM as a whole were much rougher with a different intensity and energy when he was with them.
It's very similar to comparing the different front men of AC/DC as their style changed in a very similar way from a very rough and raw sound to a more technical studio sound as their line up changed. Except of course AC/DC didn't have any choice in needing to find a new singer at the time as Bon Scott had died of alcoholism.