I found the XTerra 30's,50's and 70's to be a much more stable series than the 305 to 705's.
As Kirk has mentioned the 70's suffered far less with emi..
It seems to me that the extra processing going on in the 705 is the cause.
Even though the 30's were a rock solid cherry picker and my call for the best id 'ing unit for coinage, the added bonus of a pure all metal ground balance mode and the tonal superiority of the 305's put them miles ahead.
But with the 705 the only advantage I've found over the 70's is the prospecting mode is much more stable in hot ground and the 705's have a far better audio for that type of work with more options to utilise than the 70's when prospecting.
However, for coins I too would grab the 70 over the 705. Much easier to work with and stability with a higher sensitivity could be run with them..... meaning more depth, cleaner tone hits.