Here is the comparison Chart
http://www.gigabyte.com/products/comparison/list.aspx?ck=2&pids=4203,4315
To highlight, B75M-D3P got these while D3H do not:
Display Port Connector.
An additional Marvell 88SE9172 SATA Controller
of course, for a higher price.
D3P $91.99 vs D3H $74.99
So why pay for the additional for something you don't need? Most people who use B75 chips are for the low cost and D3H is the best choice at the moment unless you need display port connector. However, if you have the budget for a display port monitor, you would go for a high end graphic card don't you?