I agree with Dave and Paul, look at the total parts cost plus shipping.
I do a spread sheet with the part number, description, order quantity, price and extended price for each part and for each vendor. I do the research on-line at their web sites and copy and paste. It doesn't take very long. I have Moss, V-B, and I've just added LBC (Little British Car Company), on the spread sheet. I also have a local guy who is a Moss distributor. He gives a club (Green Country MG Register) discount from the Moss price (which takes care of the local sales tax). Unless there is some specific reason to purchase an item from a specific vendor, I usually go with the lowest <span style="font-style: italic">and best</span> total cost. I also tend to support my local guy, just to keep him in business should I need him <GRIN>.
While you can see a substantial price difference between vendors for a specific part (68-74 1/2 MGB BRAKE MASTER CYLINDER, DUAL LINE, NON-SERVO: Moss [Classic Gold] 180-766 $129.95; V-B 7-514 $99.95; LBC 180-765 $135.96; Range ~$35.95 plus S&H), if you go for a number of items the total cost difference, including S&H, can be very small.
I don't want to hi-jack the thread, but speaking of Moss Classic Gold brake master cylinders, I read some of the Triumph folks have had a problem with Classic Gold brake MC's for TR's (apparently, cheesy parts that don't fit/work very well). Has anyone had problems with the Moss Classic Gold brake master cylinder for the 1972 MGB roadster (as in example above)? I need to get a replacement for my badly leaking master cylinder, and I was leaning toward the Class Gold unit. I don't rebuild hydraulics very well and I've had professional mechanics tell me it's easier and better to purchase a brand new one, rather than a re-manufactured unit. PM me your response if you prefer.