I'm in a completely different field but I think that the most important factor is your career plans. For some career options a PhD is a must (for example if you want to be a lecturer) and in some industry fields it would definitively improve your career prospectives (i.e. you will have defintively more chances to be promoted if you have a PhD). So it's really up to you to decide what do you want to do in the future and analyze if a PhD would help or not.
Having said that, I think that, even if you are now determined to do a PhD, 2 or 3 years of work experience between your undergraduate and your PhD can be very useful. Work experience will be always valuable in a PhD application, you can save some money and it will help you developing some skills different to those you can develop at university.