There’s a lot to be said about practicing from the grass. It is more realistic as I said, more forgiving on your joints and gives you better feedback. For instance, if you hit a shot fat (contact the ground behind the ball), you will know instantly whether you’ve got good feel or not, as you will see the chunk of grass fly up, your swing will slow, your ball will go all of nowhere and the mark will be on the ground behind where the ball was laying. The same is true if you don’t hit down enough resulting in a thin shot. You will observe that the nice dollar bill size divot in front of the ball that you should of taken if you were to have hit that iron properly doesn’t exist.
There is a downside to grass though especially from an instructor’s perspective. For one thing, no lie will ever be exactly the same. The blades of grass, curvature of the ground, etc. will be different from hit to hit. If the lie is different, it only makes sense that the result would be slightly different, right? How then do you know if it’s the lie that changed your result or your swing? Hmmm….that’s a very good question. The other negative factor with grass is when you hit those fat shots it can give you negative feed back and cause a lot of tension and frustration. When going through changes which are difficult enough, it’s best not to add in any other causes of negative feedback to your brain.
Mats on the other hand are hard on your joints (many players myself included, have developed tendonitis from over use) and they can actually give you too much positive feedback. You somewhat get away with bad swings as the club can bounce through the mat still giving you a pretty decent result and more distance than you deserve. If you have good feel, you should be able to recognize this though.
The positive feedback and the consistent lie make mats beneficial when it comes to learning a new skill. You will be able to focus on doing the same thing over and over without too much frustration if your contact isn’t perfect.
As you can see, both surfaces have their pluses and minuses. Picking which surface to hit from is often not a choice as some practice ranges either have one or the other. If you are stuck only on mats, don’t fret is as you can see some positives now. If you are stuck only on grass I believe your practice time will be more realistic and that is never a bad thing. You just might get a little more frustrated. Any professional player would choose grass in a heartbeat, but if you are a higher handicapper and want to focus on grooving a swing change, mats are a decent choice.