G'day Barry. For a DSLR you will want no more than the 6.3. The 3.3 is meant for small chipped CCDs like the DSI and similar. If you use the 3.3 with a DSLR you will get a very unflat field around the edges. If you really want to get more focal reduction with a DSLR then you might need to consider a Nexgen 0.5x reducer.
http://www.optcorp.com/product.aspx?pid=3531
But even this won't fully cover a DSLR chip.