Hi,
The 250's are so rare ive not heard of a jet kit, if you get one you can measure the shock up and compare to the sizes of the shocks fitted to the B4's on the forum. If you can afford the extra insurance of the B4 id go for one over the 250 as being more common the resale value of the B4 will be no greater than a 250 so costing you the same to buy and if you drop the bike there are more B4's about and so more/cheaper spares. The B4 is a good bike and suits the learner well.