I have several 21's and an Umnumzaan. I do not own a Tanto, so I can't speak to the blade shape but assuming this is your first CRK, without hesitation, I would go with the Large 21. It has what made Chris Reeve Knives; and for good reason. IMO, if one is going to be turned on or off by CRK, it should be the 21, a regular, or a classic to do so. I think it would be a complete shame for someone to not like an Umnumzaan or a 25 and blow off CRK without having gotten to know the (large) 21. If you have owned a 21 before, my advice would be to go with what you think you would like to try next. The 21, 25, and the Zaan are definitely different knives from one another.
This may be a bit off topic... I am sure this goes for all of his knives, but when I look at my used and slightly abused large plain jane 21, it still blows me away each and every time what an incredible knife it is. The best part about the knife is that when I hand it to someone who doesn't know Chris Reeve Knives. I see them look at it and can watch as they realize what a simple yet complex, and SOLID knife it is. The more I use it, the more beautiful she gets.