Lucky you! I have been to Katelios twice and Skala once. When we went to Skala it was quite small, but has grown into a large resort now by comparison. Perhaps my view is coloured by the fact I had seen it much smaller.
Katelios I love, and we went in 2002ish and 2013ish. There have been only a few changes in that time. It remains small, with a good number of tavernas, several on the beachside. The accommodation is all low rise and spread out, so it feels green and airy. The resort itself is fairly flat but surrounded by hills. The beach isn't much to write home about, but there is a small harbour. There is a clifftop walk over to the next beach, where turtles nest. I am disabled and we didn't hire a car, so this is out of the question for me. If you are a Far Ender, opportunities in the Katelios area are not great. There is a bay in Skala which is/was naturist, but last time we went (on a day out from Katelios), cliff and beach erosion made it impossible to get round without swimming. I think the turtle beach may have opportunities, but don't take my word for it.
There were several buses from Katelios when we went first time, but more recently buses were extremely rare and at the wrong time. We went on an organised coach trip to Skala for about 7E per person (I think). Apart from that we wandered in the local area. We were there at half-term in May, but in high season I understand there is a water taxi to Skala and Poros and boat trips out. Otherwise, a car would be very useful.
This website has useful information and a walkabout: http://www.justkefalonia.co.uk/katelios.asp
We stayed at Efrosini, which are lovely apartments with large balconies, large rooms, well equipped with a large pool. It also has lots of cats but we managed to show them that they weren't welcome and they just walk over our patio and on the next one, giving us a look of disdain. The feeling was mutual.
Katelios is a small, quiet, laid-back resort with plenty of good restaurants and a few bars, and good walking opportunities.
On the other hand, a second trip to Plakias isn't a bad idea!