I went to Cambodia and Thailand for my Honeymoon in April this year.
It depends really on what you want from your trips. I personally found Cambodia to be a better experience to Thailand.
We stayed in Siem Reap at the Le Meridien just outside Angkor Wat (literally a few minutes ride by Tuk Tuk) This is a 5 Star Hotel but it is still fairly in-expensive compared to 5 Star in westernised countries. However (having stayed in 5* hotels all over the world) i would say that Cambodia is one of the best for hospitality and service. Or to put it another way the 5* treatment we had in thailand made the Cambodian version seem like 6* So this would be a good place to Start/End your stay in Cambodia if your after doing some backpacking. Some of the Temples are fantastic. Angkor Wat is something else and is Huuuuuge.
My advice would be to hire a Tuk Tuk driver for a couple of days. It cost me & my wife $50 for 2 days. This was him picking us up at 6am at the hotel, taking us around a few temples, taking us somewhere for lunch a few more temples and then back to the Hotel for tea. The Driver we had was fantastic as he knew the best times of day to take us to the various temples when they were least crowded. One of the places we went was a 2 1/2 hour ride away which took us through rural Cambodia and shows you just how primitive they still live outside of the main Cities.
I wouldn't recommend doing more than a week looking at Temples. Once you have seen a few they start to get repetetive. Main ones i would recommend seeing are Angkor Wat, Ta Prohm (where they filmed Tomb Raider), The Bayon, Batay Srei and Kbal Spean. I've attached some pics we took whilst there of a couple.
We also went and watched a Traditional Cambodian Dance show. Whilst good and interesting it's not something i would do again. But i would recommend it if you haven't seen it before.
In Thailand we stayed at the Evason Spa at Hua Hin. This is out of the way of the Hustle & Bustle of Bangkok (about 3 1/2 hours drive) but again 5 Star and a good base to get out and about and explore Thailand (if your not backpacking) From here we went to Kanchanaburi where they filmed the Bridge on the River Kwai, went into Hua Hin itself which had lots of traditional markets etc and is quite good for grabbing a bargain. You can get some fantastic quality dresses made and barter for the price. Put it this way i enquired about a full 100% Cashmere suit. The same standard in the UK would cost around £1500+ and it would have cost me £200. The only reason i never got it was due to not knowing how long we were going to be stranded due to the Ash Cloud. We also went out to eat a lot. This is were Thai 5* differs from Cambodian. In the hotel it was going to cost us around £50 each to have a 3 course meal. We went out 300m from the hotel and BOTH had a 3 course meal for about £12. The quality was just the same as the hotel (we got a free meal as part of out honeymoon package so ate there the last night) and you can pretty much guarantee it's fresh (especially if it's fish). In Cambodia a 3 course meal in the Hotel cost about $50 for the 2 of us so quite a difference and you can see the western influence. Also they hotel were charging nearly £70 for a massage. My wife had one done (in the evening by one of the hotel Masseuse) at a place about 500m from the hotel and paid the equivalent of £8.
Here are the pics:
Ta Prohm
The Wife by a tree seen in Tomb Raider
The two of us outside Batay Srei
Another view of Bantay Srei
To give you an idea of just how big Angkor Wat is here is a pic of the Walkway up to the outer wall (it was a City afterall)
This is the view from the outer wall to the temple (Total distance is about a mile)
This is a view from the highest point looking back towards the outer wall
Cambodian Dancing Show
View from the Spa in Thailand
Someone got bored one day and decided everyone need to know which direction to go
The two of us at the Bridge over the River Kwai
A good book to get for Cambodia is Ancient Angkor by Michael Freeman & Claude Jacques. You usually find kids selling it outside temples (in english) for $5 which is considerably cheaper than the $27.95 price on the Book
If you have any questions feel free to PM me and i'll give you my EMail. We spent a month in total over there (thanks to the Ash Cloud) and it was only 4 months ago
Carl