EJ and Muhle collaborate with each other in product design/development, so commonly they offer fundamentally the same products but with EJ or Muhle branding and with more or less minor design differences (such as different handles for the razors, etc.). Muhle products cost more in the U.S. due only to a poor distributor arrangement and thus a higher distributor mark-up in cost. UK's Connaught, however, offers Muhle brushes, including synthetics, at excellent prices. That's where I purchased my excellent HJM black fibre brush, at just $25 delivered.
As Mandina ststed, EJ and Muhle share some of their product development costs. Starting in 2013, both companies began to produce synthetic brushes using the Muhle V2 series of knots. Ejs can be found for less money for the reasons stated above. Vendors in the UK and US have good prices on them.
You are the only person that can answer your secong question. Past experience should tell you your preference. I use both the 21mm and 23mm Muhle V2 sizes, which I prefer to the 19mm and 25mm sizes. The 21mm is more precise and the 23mm is faster, since it covers more area. Both are superb.
I went with a Muhle over an EJ simply because I really like the classic olive wood handle (http://connaughtshaving.com/31h13.html). I ordered mine from Connaught Shaving, but it hasn't arrived yet.
I went with a 21 mm since 21-23 mm are the most popular sizes as far as I can tell. I don't know any better, but I went with a 21 mm as I have read that smaller may be better for face lathering and I prefer a shorter loft. But I suspect that 21 mm or 23 mm are both excellent choices.