350. I just had a 450 on my 10wt Reddington -- cast a big clouser on it for an hour and had tennis elbow for a week. [although it did go out like a rocket and sink like a stone].
Swapped it for a 350 Teeny, and I'm within 10 or so feet of casting an entire flyline for the first time ever.
The only way I'd recommend the 450 is if you fish deep pools in close, where you don't want to cast more than 20 feet but you want it to sink like a rock.