250cc Motorcycle
Pros:
A 250cc motorcycle will be faster off the line. Gives you some options for gearing with different sized sprockets to cater around what kind of riding you will end up doing. Lower your RPM at cruising speed or wind up faster for urban riding. Your wheelbase and rake will be different so it's a bit harder to navigate tighter areas, but will be more stable when going flat out. Big wheels let ya tackle rougher roads with a more planted stance. 250-300cc naked/sport bikes will let ya grab the tank with your knees and lean it over with a lot more confidence. If you ultimately want to graduate to riding big bikes, a 250-300cc motorcycle will offer the most analogous experience you'll need to jump up to 650s safely. You're up higher and you're easier to see (but remember you're still invisible), but you can also see over cars easier and what's around you. Changing rear tires is way easier! More engaging to ride, ya feel in control of everything and some people dig that.
Cons:
You'll often lose cargo carrying convenience. Lets say you're often going to get stuck in snail's pace stop'n'go traffic, your clutch hand will start to get tired. Your passenger will be less comfortable as there's not a lot of seat on the pillion of most motorcycles and can't think of any 250-300cc bikes with a decent seat with passengers in mind. Depending on where you live you may not have a lot of options. Here in the USA we're enjoying a bit of a resurgence of small displacement bikes, new/old Honda Rebel, CBR, CRF, Versys 300 (my pick), TU250 and a handful of others. But finding something you like AND that you fit comfortably in your price range is tricky. They hold their value as they're temporary stepping stones for people to learn on. When ya find a used one don't be surprised if you're the 18th owner and who knows how well it was taken care of with that many generations of noobs and their mistakes piling up. Chain maintenance is mandatory and it gets old. I ride my wife's 150cc around and even tho it's a slug compared to my Versys 1000 and V-Strom 650, not having to mess with gears and the clutch I'm actually faster in congested areas like parking lots and our neighborhood on her scooter.
Get ya a garage and fill it with all sorts of bikes, it's gonna happen eventually!