What is the size of your growing area, the dimensions? Have you picked how you will grow, soil/pots, hydro, aero, whichever type chosen do you have some growing system you want to use like SCROG for example? Are there any limitations that you know of, like very limited outlet access and or being on a circuit that has a rather low amp breaker. Is the area one that will need much if any added ventilation or is heat likely to not be an issue?
While you say you want to grow 1 to 3 plants now what are the odds that those numbers might likely change to maybe 2 to 4 or 6 to 8 plants in a year or two?
Try to think ahead as much as you can while staying within the realm of actual possibilities and their degree of probability.
Without waiting for any answer I will say that I have known few people who started out with a 250-watt HID light and did not at some point, most times half-soon, wish they had spent the extra and purchased a 400watt light. Regardless of it being called a HID light a 250-watt one really does not put off much light. To me the only time lighting under a 400-watt HID should be used is if there is no other way to grow without using the lower wattage. Only if there are limitations that cannot be gotten over or around or through and it is grow with lower wattage or not grow at all do you pick the lower wattage route.
My first HID lights were 250-watt lights, one MH and one HPS because back then there was no such thing as conversion bulbs or digital/switchable ballast’s so if you wanted multiple spectrums of lighting you purchased multiple lights. I went with the 250-watts because I fell for one of the charts that give you the size/area of the light footprint that makes you believe it can adequately fill an area up to that size with light regardless of individual situational conditional differences and because at that time grow lights not only cost more but the difference between a 250-watt and a 400-watt was rather substantial and I believed I was only setting up a fairly temporary/interim grow room and did not want to spend more on it not knowing if what I would instead purchase would later be reused when I upgraded or if I would then go higher. It seemed as if the 250-watt would do the job and I wasn’t gambling as much on what my total overall costs would be between my intended temporary grow room and the one I had planned to replace it with.
For vegging seedlings/small plants even with the pot sizes I used the one 250-watt MH was sufficient for how many plants I would grow to all fit under it and receive enough light. It was not great, but if kept close enough it did a pretty darn good job even when compared to better lighting. But it did not take long in flower to tell that I needed a second light to cover the area adequately. The chart was not applicable to the needed connected to how/what I was growing.
My brain should have SCREAMED scrap the 250-watt and just go 400-watt and you will be far better off, but it didn’t so I ended up purchasing a second 250-watt light, HPS.
I never had to hang my head in shame after a harvest but I never came close to setting any world’s records either. I was not always happy but seldom was I really amazed, either in a positive or a negative way, by a yield. For what I had to work with I did OK …. but it didn’t take long into my first grow with a 400-watt HID before I knew it was making a big difference … and the final results proved my observations to be correct.
I had made an error in my initial lighting purchase and then compounded it by doubling the error by adding the additional light when my only reasons for not going with the higher wattage light were only perceived ones and not actual ones I could not get around. It was a lesson I learned the hard way, but I learned it. It might have been the most important thing I ever learned about lighting when talking lighting generalities.
Unless some limiting factor exists that is absolute, that you cannot brainstorm and adapt or add or alter or improve or save more money or whatever to get your way around if there is ever a decision to be made between two lighting systems always pick the higher wattage lighting system. Always. Every time. Not now and then, each and every time … and I’m not kidding. Well OK, there could be some exception to that rule like some lunatic who might try to shoehorn and Vaseline a 1000-watt CoolTube into a computer case grow just to be sure to go as max as possible … so I am of course only talking about how if someone is sane and the lighting question I mentioned comes up .. .. always pick the higher wattage system.
cheers 
