I've a Pentax K-30 with the following lens: 

* Pentax-M 50mm 1.7 manual focus
* Tamron 17-50mm 2.8 autofocus
* Pentax 55-300mm 4-5.6 autofocus

Considering that : 

1. Pentax 50mm has the nicest bokeh and sharpness I've ever see but, manual focusing, is prone to error
2. and 55-300 has the 'ideal' focal length for shooting portrait (for ideal i mean 70-90mm), autofocus, but it hasn't good bokeh

Which should I use?