In the case of Fletch vs. Spikes, I'm pretty confident that if their roles were switched Spikes would do all the things Fletch does but Fletch couldn't do all the things that Spikes does at WLB. He is more athletic, just as fast, stronger, has better hands, as good of a tackler, and a harder hitter than Fletch.
With this being said, I've always felt that Fletcher was the most under-rated player on the Bills D. He is a model of consistency and has great feel for his position. He is a pro-bowl type player even though he hasn't made it there as a Bill.