I think both loops are identical:
1) number of additions, subtractions are the same in both cases (50 increments, 50 decrements, 2*50 additions and 100 increments and 100 additions).
2) number of comparison are the same (2*50 and 100 ones).
I would prefer to use the second loop, because its structure is simpler (or even just formula: sum = 100*(100+1)/2).
So then I guess it just boils down to preference. The main thing I learned was that if needed, you CAN have multiple conditions in a for loop (but you can't declare two variables in the for loop for some reason).
As a beginner, reading the second version looks simpler, but I just think the first one is a little cooler... even though they essentially do the same thing.