PC to 360 and vice versa is easier than porting to or from PS3. I don't know how much the situation improved for the PS3 but apparently it was really difficult in the beginning. I'm sure by now all the major used Engines/SDK's have gotten their shit together and improved the development experience for PS3.
It helps when the company (Microsoft) that makes your console (360) also makes a operating system that uses the same in house created multimedia API on PC (DirectX). It sure as hell helps when you've got a foot in the door.
A engine built from the ground up to run on a console or set of consoles could be a nightmare to port over and optimize for PC. A engine built to run on multiplatform from the get go like the Unreal engine makes it a ton more easier. So difficulty also depends on the developer in the end.