I use the 129 for MSF.
I only use MSF for highlighting and a general glowly look after all my make-up is done and blush applied. So after applying blush I just wipe the brush and use it again for MSF.
I have tried the 188 and it's ok for MSF but it's so small that with MSF I really only used it for a quick swipe down the nose and a tiny highlight on the apple of the cheek. If you don't already own it I might not buy it just for use with MSF unless you also want to use it with blush and foundations (which it is great for!)
If you already have a smaller blush brush, that will do the trick for MSF just fine!