I am a Simms fan. I am still trying to wear out a pair of their waders and boots I got in 2001. I can understand hesitancy to buy the best on ones first purchase, but Simms really does make stellar products.
I have to question going from a large short to a medium. Keep in mind that breathable waders don't stretch, and so you need them to fit loosely enough to allow you to squat down, lift a leg way up for climbing and various other gymnastics common on the water/trail. You may think it looks goofy to have all that extra material between your waist and ankles, but its the best way to go.
For cheap waders, LL Bean's $100 model is most widely held as the way to go. Breathable Emerger Stocking-Foot Waders, Men's: Breathable at L.L.Bean