A private guide is the last thing you need in either city, each of which you can easily navigate by yourself, unless you like being overcharged, taken to shops, restaurants, and shows that pay kick-backs, and fed false historical and cultural information, albeit often by very sweet people.

Beijing is easily and cheaply navigated by taxi (and even more cheaply and rapidly by metro). The Great Wall can be reached at various sites either by taxi, by one-day bus tour, or be direct public bus. There are direct buses to the Terracotta Warriors in Xi'an that give you as much time as you want, one-day tours that take in other rural tombs and temples, and easy access to other sights in the city by taxi. In both cases you'll find more accurate information in reading materials you bring from home, and most of the carefully adjusted accounts that a guide will give you can anyway be read on English signs at sights.

Peter N-H
China