The Seca 2 (Also called Diversion in other markets) is not a terribly light bike. It lost out in several big comparisons to the Suzuki Bandit because the seca was heavier to turn, slower, and overall simply not as “playful” as the Bandit. Not exactly bad points if your goal is an easy to learn on bike.
Speed wise I’m sure it’ll still be faster than the ninja, but it’ll be harder to ride in slow speed situations. That said, 400ish pounds isn’t _that_ hard to manhandle for someone 6 foot. But I do prefer moving around the lighter bike.
I don’t think you’ll find the seating position on it that different from the 250R, both are pretty upright bikes with a slight lean forward. Both have comfy seats.
One big Con for the Seca is getting parts when something breaks or wears out. Also, newer shop mechanics won’t be familiar with the bike, and they may make some stupid mistakes with it (Stupid shop mistakes basically turned my 84 VF500F into a paper weight). If you get the Seca, your best bet is to find a small garage the specializes in older bikes. Or get a clymer manual and do a lot of the work yourself (Between the lack of fairing, and the air cooled engine, it is about as easy as 600s get to work on).
Sounds like I’m steering you away from the Seca, but I’m not. I happen to really like the bikes, I almost bought one as my first bike – it got sold out from under me 
But owning an older bike takes some considerations that newer ones don’t, maintenance and parts in particular. If you think you’re up to it, and have a motorcycle salvage yard in your area, it’s a pretty awesome ride.
