I think you need to try them both. There are adherents to both. I "grew up" with downtube shifters. I liked them a lot. Then I bought my touring bike with barends. At first my attitude was, "I'm going to have to change these someday." But I grew to like them and now they're my first choice.

With downtube shifters you can shift both gears at once. This is handy for a sudden big change in terrain. When you top a big hill and head down the steep other side you can shift from climbing gears to full-speed gears in one move. Not a big advantage, but it's something. However, you have to ride one-handed every time you shift. Not a big problem, but if something untoward happens while your one hand is off the bars it could become dicey. With downtube shifters there's nothing on the end of your bars, so you can stick a mirror on there.

With bar ends you never have to take your hands off the bars - good if something untoward happens at the wrong moment, and good if you have a big load and are wobbly for any reason. You can only shift one side at once. (At least, I can't, or don't choose to. I suppose you could grip the ends of both bars and manipulate both levers at once with your little fingers, but that's not for me.) You can't put a mirror in the end of your bar because there's a shifter there.

My bar ends work great - smooth, never miss a shift. Click, click!

I say bar-ends for touring, but your opinion may vary.