Here is what is in each side...this is just an overview...I may have missed something:
Hogsmead...has 3 rides and a few shoppes and one restaurant:
1. The Forbidden Journey ride in side Hogwarts castle...great ride ...kind of like and overview of All things Harry Potter
2. Flight of Hipogryf Ride - a simple "roller coaster" similar to Barnstomer at WDW
3. Dragon Challenge - a great ride - a real roller coaster...has all the themeing of the Tri Wizard champion stuff
4. Three Broomsticks - Eat there and get all types of butter beer
5. Ollivanders - buy a wand - the interative ones "cast spells" in the shoppe windows
6. Honeydukes Sweet Shoppe
7. Owlry - mail a post card and get a Hogmead's post mark
8. A Harry Potter Shoppe- cant recall the name with several HP themed items...shirts , cups , ket chains, etc...the main store near the entrance of the park has LOTS of HP stuff as well
9. Moaning Myrtle might visit you in the bathroom by the Three Broomsticks
10. Several street carts sell butter beer
Diagon Ally...has one ride and several shoppes and one restaurant:
1. THe only ride is "Escape to Gringotts" and it s good, but not as good as Forbidden Journey at Hogsmead
2. Visit the bank [near the ride]and talk to an interactive Goblin and exchange your muggle money for some Gringotts money to use int eh parks
3. Weasley's Joke Shoppe
4.Several Shopps at Diagon Ally...and Nocturn Ally...one for school robes, one for stationary, one for stuffed animals / creatures...they have several shoppes...most are quite small but still very cool
5. Eat at the Leaky Cauldron - get butter beer
6. Ice cream shop ... where you can get butter beer ice cream
7. See the dragon breathe fire on top of Gringotts bank
8. Outside of the entrance to Diagon Ally you can see the night bus and visit with the night bus driver and take photos
9. The Hoping POt is a good place to get butter beer...it has picnic tables and is a good place to take a break
10. Also has Ollivnaders shoppe for wands
So...if you have to only pick one, it depends if you prefer rides to stores. Personally, Id pick Hogsmead since it has the Forbidden Journey ride inside Hogwarts and the other 2 rides. If possible, have you considered a park to park pass so you can visit both? The Harry Potter sections are fairly small and as you can see dont have many rides....