My newbie experience: I was new to dirt 6 years ago, decide to learn on a wr250r dual sport. It it was pretty good to get started but I quickly found it was not a great bike to learn pure dirt skills, it was a bit on the heavy side and had trouble keeping up with experienced dirt riders.
A year later I pulled the trigger on a KTM 200 two stroke. The difference was amazing! Light, agile, that sweet sweet power band :teeth. Took me a long time to adjust but after a few years or riding I could keep up with some fast dudes. This is the bike that I learned all my dirt skills.
At some point I upgraded the wr250r and moved on to a KTM 500 EXC plated. It was definitely an improvement over wr250r but a step back over the 2S. The 500 was heavier and less agile than the 2S but had a plate, so I could ride Death Valley and other places I cannot take the 2S. Plus I got sumo wheels and ride the crap of it at the track :ride
Today I still have the 200 and 500 (and a few more... they keep multiplying! :laughing). Looking back, if I were to do this again, I would still aim to have a 2S and a lightweight 450/500 DS. If I were to have only one, I'd go for a lightweight performance 250 DS (KTM 250 EXC-F or any non-orange brand) and have the best compromise between a 2S and a powerful DS.
Good luck!