Average grade: (70+90+80+80+80)÷5=80,

Median: arrange this set of data in descending order: 70, 80, 80, 80, 90, and you will get a median of 80.

Mode: 80 appears three times in this set of data, the most times, so the mode of this set of data is 80.

Variance: S2 = [(70-80) 2+(90-80) 2+(80-80) 2+(80-80) 2] = 40;

(2) In these five exams, Xiao Li's performance is relatively stable, with excellent rate of 40% for Xiao Wang and 80% for Xiao Li;

(3 Option 1: I choose Xiao Li to participate in the competition, because Xiao Li's excellent rate is high, and he scored 80 points four times, and his performance is relatively stable, so he has a great chance of winning the prize.

Option 2: I choose Xiao Wang to participate in the competition, because Xiao Wang's performance has a higher probability of winning the first prize, and both times are above 90 points (including 90 points), so it is possible to win the first prize.