If it were me, Cambodia it is, hands down. I wish I would have brought my pics into work because you wouldn't have even posted this poll if you had seen them. I can't say Phuket isn't worthy but we loved Cambodia. The must sees in Cambodia would be Angkor Wat in the north and Phnom Penh in the South. Three days in Siem Reap and Angkor Wat would be the minimum I would recommend. You could take the backpackers loop that would get you back to Bangkok or a quick flight in and out of Siem Reap would do the trick too.
We went overland by bus from Bangkok to the border and then hired car from there to Siem Reap, a flight would save you some time. Once you've seen the wonder of the Angkor ruins (think Tomb Raider) you can take a 1/2 day boat ride down Lake Tonle Sap right into downtown Phnom Penh. Once there I recommend a full day spent at the Tuol Sleng prison, now genocide museum, and the "Killing Fields" of Cheung Ek just outside of town. Be prepared for a sobering day of revelation into the genocide of the Khmer Rouge and Pol Pot. There are many great restaurants right on the banks of the Mekong river to dine at. After a couple days in the capitol you could head west to Sihanoukville by bus for an overnight stay or an immediate boat ride back to the north border crossing back to Thailand at Khlong Yai. From here a mini bus can get you to the town of Trat where we caught a ferry out to our favorite island Ko Chang where we lounged for a week. If you do somehow make it to Ko Chang I recommend a stay at Cookie Bungalows, little one room buildings A/C or not, right on the beach. After relaxing for a week we took a bus from Trat back to Bangkok and continued our journey in SE Asia.
Plane trips into and out of Siem Reap/Phnom Penh would save you a bit of travel time but some of the legs of the journey like the rooftop boat ride down the lake are fascinating looks into the lives of the Cambodian people that live on the banks of lake/river.
Good luck with your decision!!