You can probably look at the GPU database compilation https://www.overclock.net/forum/55-o...-database.html
A R9 270X is about 96K PPD per the chart so it's not particularly power efficient as it was rated 180W TDP. It's predecessor HD 7870 GHz Edition it was based off had 175W TDP and the chart lists that at 78K PPD. Maybe with an undervolt it will be better. Pitcairn is generally most efficient around 900MHz - 1GHz.
At a minimum if you're looking at 1080p 60Hz I'd look into the RX 570 4GB or better (Vega 56 is the next tier for double performance), GTX 1060 6GB or better on the Nvidia side (GTX 1070 Ti / RTX 2060 for double performance compared to that).
The RX 570 is revamped RX 470 so with some undervolting it should be below 150W (miners get it under 100W but that isn't as core intensive load) while putting out 267K PPD. It can typically be found for around $120-150. RX 580 8GB is an option, but the price needs to be below $200 to be worthwhile (it's about 10% improvement over RX 570 / RX 470) and it's a power hog out of the box at ~ 185W TDP.
The GTX 1060 6GB is EOL and now replaced with GTX 1660 Ti 6GB GDDR6, if you can find it (the GTX 1060) on sale for < $200 it's a decent option though as the GTX 1660 Ti runs around $300. It's a 414K PPD card , with 120W TDP target power out of the box for reference based designs typically.
If you intend to use 120+HZ or 1440p then I'd go with the RTX 2060 6GB GDDR6 (160W), Vega 56 8GB HBM2 , or GTX 1070 Ti 8GB GDDR5 (180W). The Vega 56 is less power efficient generally despite being a 210W TDP GPU it tends to draw more unless you undervolt it heavily.
As for specific GPU models within a brand: look for proper VRM , memory, and GPU core cooling and powerstages that aren't cheap mosfets.
I know EVGA has had a fiasco with some Pascal cards' VRM cooling , although their warranty is great.
On the AMD side, people swear by Sapphire Nitro+ (at least on Vega and RX 500 series).
Reference this most recent chart as well:
edit: see also
Card Cost Yield TDP Value Efficiency
US$ kPPD W kPPD/$ kPPD/W
RTX 2080 Ti $999 2400 260 2.40 9.23
RTX 2080 $699 1400 225 2.00 6.22
RTX 2070 $499 1275 185 2.56 6.89
RTX 2060 $349 1050 160 3.01 6.56
GTX 1660 Ti $279 750 130 2.69 5.77