I have not used the A6100 but, I have considerable experience using the A6400. I would definitely recommend this camera in lieu of the A6100. The price difference is about $150 USD and, IMO, that amount would be well spent to get the added capabilities of the A6400...
If the extra $150 is not within your budget, I would get a used SEL 50mm f/1.8 OSS lens to match with the A6400. The 56mm f/1.4 lens runs $429 at Adorama and you can get a used SEL 50mm f/1.8 lens for well under $200 on eBay. The SEL 50mm also has OSS which, since the A6100 and A6300 don't have IBIS might be an advantage in sooting in low ightsituationes.
E 50mm F1.8 OSS
This image was done with the A6500 and SEL 50mm f/1.8 OSS lens. I have replaced the A6500 with the A6400 because of the vastly improved Eye-AF...
The A6400 has a better viewfinder than the A6100 and the buffer is larger. If you shoot video at all, the A6400 has some advantages. The A6400 also has a slightly better build than the A6100...
I’m looking for a low cost combo For posed portraits. In terms of eye AF and sensor and image quality aren’t they the same? I know the A6400 has more fine things but right now A6100 is $598 which is why it’s very tempting.
thank you for replying
I had not kept up with the price reduction of the A6100. At $150 less than the A6400 - I would choose the A6400 but, with the extra $150 off the A6100 making it $300 less than the A6400 - I would choose the A6100...
I still think that if you are really wanting to save money, a used SEL 50mm f/1.8 OSS lens might be a good choice at less than $200 used on eBay... That would make it $798 for the camera with lens...
The a6100 has the same sensor (24MP APS-C sensor and Sony's latest BIONZ X image processor), AF, and real time tracking. For IQ, AF, and tracking, this is an excellent camera. Unless you need the whistles and bells, it looks like a nice package.