Bob,
I have an 8" and a 16" for my Woods 7500 backhoe. I have used the 8" bucket for trenching, cutting roots and ground prep for laying 6" x 6" landscape timbers. I've used the 16" for transplanting shrubs and general excavation.
I suppose the 13" would be a good compromise, but I have to side with those that recommend both narrow and wide buckets. It definitely increases the usefulness of the backhoe. Considering the cost of the hoe, another bucket is just another couple hundred bucks. (That's much easier to say when it's someone else's money.) You might be able to work a better deal at the time of purchase. My 16" came with the hoe. ( I think Woods includes the price of any size bucket with the hoe.) I believe I paid about $300 for the 8", which included three teeth. I bought the 8" about one year later. The buckets are made by Gannon, sold by Woods.
It takes less than 10 minutes to change buckets, and that includes the time necessary to get the tools and regrease the pins.
Just my $0.02 (and your couple hundred bucks).
Good Luck,
~Rick