I'm a huge SM fan, one is always in the garage. Great bikes for their specific task.
The 300 stock is geared way too short and the power falls flat at around 10,500. Geared +1 CS it isn't tall, just normal. For long distance it can easily pull more gear, and a tune puts the power out right to the soft limiter at 12,500. Then it is a screamer. As delivered it never needs to clear about 8k for keeping way ahead of normal traffic.
The new engine, IMO, puts it right in the park where the old Ninja 500 was, but it has a wider gear spread by my estimates as I had to drop the EX500 gearing to be acceptable in the role of commuter/back roader/touring bike.
We had a WRR in the fleet for a year, my 300 would walk it stock and I'm just bumping into the MSRP of a WRR with my many additions. Nice bike but for sport touring?
Comparing new/used is a lost cause. I wanted a sporty bike, as light feeling as possible, to carry 2 up and luggage. I could have a SV650, but it feels like a tank compared to the 300. I just rode my buddies last week. For a first bike, get the 300 over the 250. It has every feature of the 250 to make it a good first bike + much more useable torque. If it bores you, sell it of make it better. It responds extremely well to simple mods.