I agree with Ken on one point at least.. if you think it'll bother you, now IS the time to make the move. My
L3400 is better than some, worse than others. I don't do a lot of precise grading with it, and I like the tractor otherwise. If I could fix it cheaply, up to about $500 out of pocket, I would. Any more than that and the improvement wouldn't be worth it to me.
I think my approach would be to get the best price you can out of the dealer for an "upgrade" to a model with a smooth hitch, then sit down and think for a couple days whether the improvement is going to be worth X dollars over the long run. Then make your decision. $5000 or more is a lot of scratch for a smooth hitch if the MX is a good fit other than the 3pt.
I think I'd be pissed if I wasn't offered my money back. After all, you committed to buy a Kubota costing $25K, not one for $30K+. If the $25K tractor isn't suitable and you weren't aware of the hitch issue when you bought it, then why should they be able to leverage you into either keeping it as is, or force you to spend more to upgrade? I know they've tried to make it work, but it just doesn't feel right to me as a way of doing business. If the dealer thought there was nothing wrong with it, they wouldn't have tried to "fix" it.
Put it this way, if you had $25K to spend and you knew Kubota didn't have a 51 HP tractor with the smooth hitch you needed for $25K, you would have had a Deere or New Holland in the barn, as an example.
I don't want to seem unreasonable, but I think it's a poor way for a company to do business.
Sean