The type of toilet, round or elongated, has nothing to do with the rough-in dimensions. At 11.5", since MOST, but NOT all toilets have about a 3/4" gap behind them (you MUST check the model's specs to be sure it would fit), you can use most any 12" rough-in toilet, and have a smaller gap behind the toilet.
An elongated toilet is required in public restrooms, and is preferred by most people. So, unless there is a knee clearance problem go with one of them. If you're looking for a generic recommendation, the Toto Drake is a robust, reliable, well-working toilet that is moderate in price. It is available now (and may become the only version) as the eco-Drake, which only uses 1.28gpf, rather than the 1.6gpf. Jim
The sitting position is the same for elongated or round. Your knees will be in the same location.
The elongated is just a little longer, at the end of the bowl. added by Terry
http://www.easycarts.net/ecarts/terrylove/Toto_Toilets.html#gmax_flapper