Here is a statistical comparison between them.
Alshon Jeffrey60.8 rec., 910 yards, 5.2 TDs over his first 5 years. (He produced this with missing 17 games over those 5 years too.)
Projected stats this year: 63 Rec., 900 yards, 10 TDs.
on the injury front, he's missed an average of 3.4 games per year.
Sammy Watkins51 Rec., 820 yards, 5.67 TDs over his first 3 seasons.
Projected stats this year are 41 Rec., 713 yards, 7 TDs which is a tad below his career averages.
He has missed 11 games in three seasons for an average of 3.67 games per year.
Conclusion:(1) Jeffrey's has been more productive overall; Watkins has a slight edge in TD production.
(2) They are about even on the injury front with both missing significant time.
(3) Jeffrey's has a big edge in recent production - 2015-2017 - with at least 800+ yards per season despite missing 11 games in that time period; OTOH, Watkins has had a couple of subpar seasons in a row now. Albeit, this year with the Rams, his contribution to their explosive offense exceeds his stats given the attention on D he attracts.
(4) Watkins has a big edge in the age category being three years younger. He is 24, Alshon is 27.
I don't see the age thing being enough to overcome the other advantages to Jeffrey's and put Watkins in Jeffrey's category especially given his recent suppressed statistical production. Based on this Alshon Jeffrey's contract, I'd estimate that Watkins will get something in the
11.5M range.