I have recently acquired a 149 and a 146 - both new from Fountain Pen Hospital.
Buying Experience:
What a wonderful shop. If you live in NYC, and are considering buying a pen, I highly recommend Fountain Pen Hospital. I have to say that I made some wrong choices in Nib type that I wanted. I ended up buying an M but for some reason I wasn't satisfied with its smoothness (a little scratchy). I took it back to FPH and the gentleman who owns the business replaced it for me. Of course, you should be much more confident that you really like the pen before inking it. It helps to not ink unless you know you like the pen.
I have good things to say for FPH. I have bought pens from them in past, and they are a wonderful shop. (I am just a happy satisfied customer). I do have to say that my buying experience felt a little rushed, and they should offer more advice and let the customer explore more nib types etc. or multiple pens from same nib type - but overall I am very satisfied with their level of service. And also their prices. A very Good fountain pen shop.
They get A in customer service.
Nib Sizes:
As I previously noted, I initially went for an M (and also tried an F). However, it seems M and F are not for me. For some reason, I don't find them smooth enough. It might have been my particular pens that I tried, but B seems to be much more smoother and elegant to me. Of course, it depends on application and personal choice. But I am curious to know if as a general rule, Bs are more smoother than M or F. I have heard that the so called 'sweet spot' is larger as you go toward broader from XF to B.
Writing:
Both my 146 and 149 are B and both write buttery smooth. In my personal opinion, you should not be satisfied with a FP unless you are really satisfied. And definitely do not ink unless you are sure. Both my MBs write buttery smooth - I think 146 is smoother (but that depends on the nib). My initial experience with an M wasn't pleasant at all. It caused me a lot of anxiety (I am a bit impatient types).
Looks:
Both 146 and 149 and beautiful pens (as the above review has already noted). 149 is of course bigger than the 146. At first look, it didn't seem that attractive to me. In fact, every time I hold 146 it feels exactly the right proportions. However, 149 grows on you. I find myself appreciating its beauty so much more often now. By all standards, it is a very beautiful pen, and to me feels like a very well made pen.
The precious resin on MB has this very subtle reddish tone when held in direct sunlight. The barrel is actually very subtly translucent and has an amazing shine to it. I have some MBs from a couple of years that have maintained their shine just like new.
Balance:
Both 146 and 149 fit my hands like a charm when unposted. Initially I didn't feel that much difference between 146 and 149. Now however, the 146 seems much more sleeker, slightly faster and meant for fast work. 149, on the other hand, seems to be slightly heavier (and is bigger), as a result of which, it is a bit of extra work to run it as fast as the 146. However, it's not too slower - just a little heavier and bigger.
Both pens are very well balanced when unposted. Feel absolutely natural. Posted, however, both pens become heavier and bigger - and slower. I don't particularly prefer unposted over posted because depending on writing style both modes seem to have their own benefits. Unposted is faster. Posted gives more elegance to the pen and just looks better and more special.
I have to say that 149 posted is the best looking pen I have seen among its peers.
Comments on MB:
While this is a very personal topic, I feel that MB pens are very well made. I find them to be consistently high quality, very fine instruments.
However, I think they should do more work on their nibs. With today's technology, the nibs should be so much more smoother.
So far as hand crafted goes, they can hand craft all they want on the entire pen (including the nib). However, the tip - which really is a big part of the whole writing experience, should benefit from all technology innovations, including consistency which is achieved through automation. I think a 600 dollar pen's tip should be smooth like butter.
Nevertheless, all the Bs that I have tried seem to be super smooth, so maybe I don't know exactly how a smooth M or F should be. My personal experience with broad MB nibs has been very positive, just as with M and F hasn't been that great (However, I might have my expectations too high - I doubt it though).
Ink Flow:
All my MB pens seemed to skip right out of the box. However, All of them started giving great consistent ink flow once I flushed them with cold water and re-inked them. I think there is some amount of break-in period with these mont blancs. Also, it depends on the ink.
Overall:
If I was to choose only one pen, I would go for the 149, just because the bigger nib size, and pen size gives it a more royal feel. Both pens are extremely well made and very beautiful. They both have a simple elegance that is hard to match.
I am in love with both pens.
If you haven't tried a MB pen before, I would recommend trying a 149 with a broad nib.

