Since the output is web, RGB is choosen, but is there any difference between "Monitor RGB..." and "sRGB..."?
In Photoshop, they are both under
color settings > working spaces > RGB
They can be the same, or at least appear to be the same, depending upon your monitor calibration settings.
The Monitor RGB setting uses the calibration profile for your monitor. That may be sRGB or not. You could easily calibrate your monitor to be wildly different than sRGB.
The sRGB setting uses sRGB.
sRGB is what most PC's and monitors use and it will display reasonably well on emails and web pages without the need for any color management software (web browsers and the like do not offer color management). While sRGB is generally well matched for your average PC monitor, the "container" is rather small with this color space: it doesn't cover some of the more vibrant and saturated shades that might possible to capture with the camera and reproduce on your printer. That brings us to Adobe RGB. Adobe RGB is a larger color space than sRGB, meaning that the container is large enough to hold colors that would be "clipped" in sRGB space due to those colors being too bright/saturated to be reproduced in the smaller sRGB container. Shooting/storing images in the Adobe RGB color space will allow you to capture and therefore later reproduce vibrant, saturated colors like deep yellows, cyans, and magenta colors found in subjects like flowers, some clothing dyes, and other subjects with very deep and saturated color.
for more details check here http://www.steves-digicams.com/knowledge-center/in-camera-color-spaces.html
sRGB is the web standard, you should/must be using this color space.
Monitor RGB can be what ever you monitor is calibrated to. There for if you have calibrated your monitor you should have it in the proof settings so you can proof all your colors as you work.
For example, sRGB on my computer lacks some red in Photoshop, so it may look fine till i proof it, then I realize its extra red every where else as well. So i proof and tame the red.