I think that you should go with the new horse. You will have so many more possibilities to excel in your riding and you will learn new stuff. The new horse can probably teach you more than your horse now.
Well I guess it depends on wether you want to progress or not. If you want to progress and learn more and possibly do shows and stuff I'd probably go with the other horse.
But if you don't mind where you are now then just stay with your lease horse that you have now