I agree with Ranto above. First of all it is never too late to go to school. This will be entirely based on what you want to do in life.
There are differenes with between a PhD and MD. However if you want to be a pediatrician or practice medicine then you will want to obtain a MD. Depending on whether or not you wish to do research as a pediatrician then you may want to do a MD/PhD program. Either way in order to practice medicine you must obtain a MD.
As to differences:
You cannot necessarily say one is harder than the other. They are different. Doing an MD can be very physically draining. There is a lot of information that most be crammed into your head over the entire time of study. Generally this will take you four years. MDs work generally with the "known".
A PhD doesn't necessarily cram information into their head over the entire time of study. They take about 2 years of mandatory coursework and then spend the rest of the time in the lab. It involves creation of knowledge and critical thinking. PhDs work generally with the "unkown". It can take anywhere from 4-8 years to obtain. You do not "need" to get a masters to obtain a PhD. This is something that will depend on the school and program you are in. There are PhD programs in which you just do a PhD and don't have to do a masters. Often times if you fail out of a PhD program you will just end up with a masters being given to you (as long as you can fulfil the requirements).
Time:
Another thing to consider is that after you complete your coursework (4 years) for an MD you will have earned the title Dr. You then go off to do residency and then get certified.
This is different for PhDs. You don't get the title Dr. until you finish your coursework, research, and pass an examination. This is not unlike doing the residency and certification BEFORE you get the title. Hence PhDs can take as long a 8 years (but can be as short as 3-4 years). After all research is not necessarily predicable.
Again doing a PhD or MD should be based on what you want to do and your type of personallity. If you are good with critical thinking and like research and creating knowledge then a PhD will be a good fit. If you are good with holding massivie amounts of information and recalling it and of course want to practice medicine (and hopefully good with people too) then an MD is good for you.
Money:
MDs on average make more than PhDs. HOWEVER, a MD will have a lot of bills to pay off after schooling (and med school is not cheap).
PhDs (often in the sciences and engineering) will have their schooling payed for by the university AND will be payed a stipend throughout the schooling process. So PhDs often can finish school with only the debts they had from undergraduate study.