Personally, I use a Thinkpad T420, but it's old and I honestly don't really recommend the Thinkpad line anymore unless you're buying refurb (which I do highly recommend). I'm not super familiar with modern laptop lines, but here's what you should be looking for, in order of importance in a debate laptop.
Battery life. There is absolutely no guarantee that outlets will be readily available at tournaments, especially if you're in High School. Not being tethered to a wall is kind of a big deal, and not having to worry about your laptop dying on you mid-round is a bigger one. If getting a slice battery is an option, you should look into it.
Durability. Debate laptops take a lot of abuse. I have yet to see a debate laptop without some sort of physical damage. You're going to spill shit on your laptop, and drop it, and your partner's going to pick it up by the screen like a moron every damn time they give a speech no matter how many times you insist they do otherwise. If one of these leads to your laptop breaking, you're boned at least the rest of the tournament. Don't let it happen to you.
Portability. Having additional weight in your backpack kind of sucks. Don't ruin your spine for debate. It's not worth it. Also, there will be rooms without podiums. You want something you can read off while holding it in one hand. Particularly big laptops may even necessitate carrying an extra bag if it doesn't fit in your backpack, and that's one more thing you have to worry about leaving in a round.
You should probably prefer PCs to Macs because the templates I've seen work better with the PC version of Office. Also, if a matte screen is an option, get one -- it makes reading off the screen substantially easier. Most of the spec-y stuff (RAM, CPU, Video Card) is a non-factor when it comes to debate. If you're buying pretty much anything made in the last 7 years or so, the specs should be fine (and that's a conservative estimate).