The first difference is size. The 900 is good for 1 1/8" breech with touch hole centered on the barrel flat. The 1700 is maybe usable for a 1", but it would possibly look too small. Many call it a pistol lock.
Second, the period depicted different. The Manton may be 1770 or so (England) in style, whereas the 900 is probably 1815 and later in England, perhaps up to '30 and '40's in the US. The Manton is something post 1790's in US, but is build a bit fancier than most export locks would have been (e.g. the hidden screws on frizzen spring and pivot). That same goes for the 900, though it is not as nice as a highend British domestic lock, but a lot nicer than many exported to US, also a bit larger.
The internals are similar in concept, but completely different parts. The real "big brother" of the 1700 is the so-called "Durs Egg" (1100) -- style is very close between them, main difference is size. I have a Durs Egg and a friend has a Manton (on a slim rifle) -- they are hard to tell apart, but one is much bigger. Both of those work pretty well.
Functionally, the Manton is very fast due to small size and gets very favorable reviews. The 900 gets more mixed reactions, though I'm sure any good lock tuner could make a screamer out of it also. It is one of the locks I have my heart set on trying, just for the looks, even if it takes me a month to get it working the way I want.
Just as an aside, I think Bob Roller still makes a modified version of the Manton and/or Durs Egg. If it fits your budget and those styles fit your build, you wouldn't have to worry about quality with one of his.
Also, you might want to look at the Chambers Late Ketland. Stylistically it is somewhere between the Manton and the 900, and a little plainer. One drawback is that there is no "exact fit" cap lock replacement for it (there are for the L&R's), which might or might not matter, and it has no left-hand version.