If your Virtual machines are also accessible by others in any way, using the virtualbox built in RDP server instead of the windows onboard TS system is an effective attack surface reduction, you also only have to remember 1 ip or hostname, as all RDP requests are sent to the host rather than to each guest. so your access becomes more centralized.
RDP login can be setup as single login authenticated by virtualbox itself, can be setup for no login granting faster access, but less security, or even with other authentication including windows account login, which allows virtualbox to fit right in with an AD-DS enabled network. it can even grant you RDP access to a guest that normally by AD or group policy would have its rdp server demon disabled.
the windows onboard rdp however, is more compartmentalized, so if you must grant others access to a machine via rdp, there is less chance of them accessing a different machine then what they are authorized for. onboard rdp also provides quicker access to client side resources, such as hard drives and printers.
the best bet is to read the virtualbox help file on rdp, and then read the microsoft technet article for rdp and compair the features each offer, with your needs.
https://www.virtualbox.org/manual/ch07.html#idp5900528
http://support.microsoft.com/kb/300698
http://blogs.msdn.com/b/brendangrant/ar ... about.aspx
however both setups have there pros and cons, and your needs dictate what method to use,
you could enable both RDP in virtualbox and in the windows guest, and play with there features a bit, see which one fits your needs best, then simply turn off the other.