Lets start with Siem Reap - a driver of a tuk tuk or car is not usually a licensed guide and will not be allowed into the temples. He/she may be very knowledgeable and help as to what temples are best seen at the best times to avoid the main tourist crowds (there will always be lots of people except at some of the outlying temples) What one usually does is hire both a driver and a guide for at lest part of the time. A good guide can make all the difference in explaining what you are seeing. A tuk tuk will cost $10-12 a day and an a/c car (highly recommended) around $20-25. A guide is about $25. Slightly higher for those temples further away. You do not pay any entrance fee for a guide. I don't know anything about the Mandalay Inn but the location is good. And yes the town of SR is easy to walk around. Be warned that even in the cool season it is hot and humid.

In Phnom Penh it is best to hire a tuk tuk (or a/c car) for a day to see the main sights but you do not really need a guide here. I would not call PP a easy city to walk around, the main streets are okay but get off them is entirely different - not unsafe but not that pleasant. The best is along the riverfront and do stop at the famous FCC club for a cool one. To understand some of Cambodia's past history visit the Tuol Sleng Museum and the Choeung Ek Killing Fields. Can be very disturbing so be warned. The Cara seems to be in a fair location. You might also look at the Billabong Hotel (close to the bus station) which have be gettin good reviews and it has a pool. Around the same price.