QUOTE
Hello, according to the charts shown the 150 and 200 do take the very same nibs, and the 150 also takes the 100 nib, unless I am overlooking something.
http://richardspens.com/?page=sales/newp_pel.htm#nibchartHawk

I don't think they take the very same nibs. I'm looking at the nib chart that you linked, and I interpret it to mean that the M1xx nibs will fit into M1xx and larger pens, but an M2xx nib will
not fit into an M1xx pen. So regarding what Leizl said, the pre-ground Pelikan specialty nibs by Richard, which are the M2xx size, will not fit into an M150 pen. So the M150 size would be limiting if you wanted to dabble in those.
Regarding what Taki said, you wrote this:
QUOTE
Hello Taki, I simply don't understand how any can notice the size difference, there is only .05 ounces different in the weight or 4.2 grams. Size wise the 150 is only a tiny 5mm difference in length, in the dark writing with either pen I would wager one could not tell the difference between the two.
I think what Taki means is that she noticed the difference in barrel diameter when she mentioned that the M200 size is more comfortable to her than the M150. If you scroll towards the bottom of the nib interchangability link above, the specs show not only a difference in weight, as you said, but also a difference in barrel diameter. What that suggests to me (though not stated in the specs) is that the grip/section is also slightly narrower. It may well be that the difference is unnoticeable to some folks. I've never tried an M150 so I can't say, but I do know that there are certain ballpoint pens that I find very uncomfortable to use because they are narrower in the section and/or the barrel than my usual fountain pens (and, yes, I would be able to tell the difference--if writing in the dark, as you say-- between a too-narrow pen and one that is more comfortable to me).
But if you're perfectly happy with your choice of the M150, I'm not here to dissuade you. I'm just trying to clarify what others have said, in case those issues matter to you. I'm sure it'll be a lovely gift, whichever you choose.