Phuket is still a great place to come and vacation. The food is good and it's relatively safe.

You know you're at the busiest time of year. What will that mean to you? Busier beaches, restaurants, hotel pools, and terrible traffic jams.
The rip off taxis and jet ski scammers are all still there, but that doesn't mean you can't avoid dealing with them and have a great holiday.

As for the differences between Surin and Karon, they are very different.

Surin has a small bay, which means it could be very crowded during your visit. Karon beach is appx 4km, so even while it's busier and has more people, the beach has a lot more space.

Otherwise, Surin is much more condensed. Going anywhere but the beach in Karon means a greater amount of travel/walking just because the grounds are so vast, it takes 15 minutes of walking to get to the next block and off the property.

Surin has a little bit of nightlife (or so I've heard), and so does Karon. Any real late partying would require a taxi to/from Patong either way.

If I was to choose, I would stay at Surin, unless the small & crowded beach will put you off. The points that you've booked is just such a great value.
The Hilton is a great hotel. I've loved all my stays there, EXCEPT during Xmas/NYE week as the lounge was too crowded to enjoy. The beach is LONG, but so are the walks in and out to get a bite to eat off property.

No matter which hotel you choose, come and enjoy.
Remember to be careful about the rough/dangerous rip tides at both of those beaches (even with strong swimmers, during the rough seasons).