Quick prime comparison: help

Just wondering about a little comparison for anybody with the know who on these here. The LDT 43mm 1.9 ($584) http://www.bhphotovideo.com/c/product/409200-USA/Pentax_20180_SMCP_FA_43mm_f_1_9_Limited.html , or any of the other 50mm options. Specifically I have been looking at the FA options as they have bother the aperture ring and will be a better investment should pentax drop us a FF. The F50mm F1.7 can be had for under $200 and has really nice reviews on "the other pentax forum".

I am leaning towards the 43mm but if I could settle my mind on something cheaper it would be nice since I'm also likely to grab the FA100mm 2.8 which is at least $400. I really like the charts I see on the DA50 1.8 but as I say, FF lenses seem a better investment. Any advice would be cool, even if it's that they are all just about the same.