I know several guys who make great cider, which I do as well, and I also make apple ale. But I don't know anyone who uses whole apple slices in secondary. We all use juice. One guy uses his uncle's apples and juices them himself, and it turns out great. They are various varieties. I too have juiced my own, based on whatever I have on hand, which has included Cortland, Mac, Honeycrisp, Gravenstein, Wealthy, and many many others. To be honest I don't think it much matters what varieties they are. The real point is that you get the freshest juice. If you are fortunate to live near several orchards as I do, you can pick up a gallon of different ones from several different places, taste the sweet ciders side by side, and there will always be one or two that stand way out from all the others as having a lot more apple flavor, more or less acidity, etc. I now go to the same guy every year because his juice is always the best and even better than I can juice myself at home, and I've asked which varieties he uses and it's much the same as mentioned above: Cortland, Mac, Gravenstein, etc. because those are popular around here. Elsewhere in the nation I might get something else but like I say, it doesn't much matter as long as it tastes fantastic. If you use mediocre juice, you'll get mediocre hard cider. But let someone else do the dirty work themselves! Just buy the juice. If you don't live near an orchard, your grocery store probably has one or two preservative-free juices that you can use. I know the national brand Simply Apple has no preservatives and is ~pretty good. I've actually used it a couple times for topping off the fermenter.
In any case, also, be aware that any apples will add quite a bit of tartness to the beer... so I wouldn't choose a very tart apple on purpose because it can turn the beer downright sour. Try to pick ones that are more on the sweet side if you can.
I know I'm avoiding your question about "yeah but can I use whole slices?" because I don't think it's a great idea. They'll take up a ton of space, leave a ton of sludge in the beer. You'll lose like half the batch being unable to separate the beer from the sludge, OR they won't add any flavor at all but just a hint of tartness to the final beer.