Both 991ES and 991MS are good and perform all possible operations for Engineering Mathematics.
You have quite good advantages in 991ES. Its like you can view decimal point numbers in fractional form.
eg: 1/2 instead of 0.5
This is a good advantage when u need final answers in fractional form.
The above mentioned feature is not actually a disadvantage for 991MS. Its just a loss of insignificant advantage.
Also 991ES does matrix operations quite easily and very much user frienldy.
I have used 991MS anyhow.
But i suggest you to go for 991ES for some insignificant add ons as mentioned above.
All the best.