There are differences between newer unibody cars and older body on frame cars and trucks. Learning the newest techniques means you're employable anywhere and gives you experience with panel repairs and thin metals, etc. Lots of older rigs used 18 to 16 gauge steel instead of the lighter stuff used now and the thicker metal behaves a little differently too. There are normally more rust issues with older cars too, so getting experience with smaller-scale repairs gives you an idea of what replacing a half or even whole floorpan might entail. Another aspect of older vehicles is you can't always just order up a new piece of body structure, often times you will need to use an English Wheel, bead rollers, and planishing hammers to fabricate what you need. I would look at it this way, get the job that pays your bills first, then hone skills that lead to working on old cars. And realize "restoration" is a precise process, I'm not restoring the 55 F100 I have, it would be too expensive and it wasn't intact enough to start with anyway. In a restoration shop, you'll be doing more tasks than just bodywork normally, so it's a more involved way of working.
Once you get established, you can do what I'm doing and hot rodding an older truck or fat fender car, etc, when you start that you can see the differences in what shops do. Some shops simply do rods, some do restorations exclusively, some take everything, but in the long run, you'll find a good balance and find your niche.
I learned airframe/aircraft sheet metal instead of auto stuff, some processes transfer, some don't, like welding on a car instead of doing multi-level patch repairs on planes.