KTM 250SX vs 350SX-F for Vet Novice

The 250SX can be a handful but is way more fun to ride.

The 350SXF would be a much better bike for racing, but not as fun IMO.

The weight difference is noticeable from the 150 to the 250 to the 350 also.

Seems to be an easy decision. Choose the 2 or 4t power style you prefer, and from your pure enjoyment is sounds like it's probably the 2 stroke. I have both the 150 and 250 and while theres no doubt sheer enjoyment goes to the 150, the 250 is still a fun style of power. Use the power valve and/or fly wheel weight if needed. As a vet B rider, I personally dislike adding more flywheel weight as it takes the playfulness out of the bike. That may be fine for offroad, but for moto it sucks IMO unless super dry or muddy, but then I don't ride in those conditions anymore either (fair weather rider now)

On a related note, the newer generation 250's have much less top end than the pre-counter balancer generations. Some people like to argue this point, but it's true. While I love the smoothness and frame of the new generations bikes, the previous generation had a better broader powerband for the typical rider. In addition, the new generation internal gear ratio has a very low second gear and the spread between 2 and 3 sucks, and it cannot be corrected with external gearing. And to top it off, the Mikuni carb is much more finicky and the jetting specs in the manual are way off.

The new generation 250 is a flawed bike in many ways, and that comes from extensive riding on both current and previous generation 250's. It's the reason I'm still hanging with my 2011. For more confirmation, just read the latest review of the 2019 250sx from MXA, it's obvious they have experienced many of these similar concerns. Many riders still love the new generation and the benefits over it's shortcoming, so it's not the say that it's not a very good bike. Just know the awesome overrev the 150 does not exist when moving to any 250, but the older gens have a much less dramatic loss.