There are some real similarities between the two. Both Shane and Merle had strong fan followings despite being bad guys. Both died on the second last episode of the season and both turned into zombies!
it kinda surprised me how merle decayed so fast, i mean, i'm not actually sure, but it looked like only a couple of hours passed between his first death and daryl finding him zombified eating a corpse. now that you mentioned it, it is true, both were really similar, i never really paid attention to that. well i did hate shane, and merle was a dick, a racist and reddest and trashiest of all white trash rednecks, but he was more....likeable than shane for me. maybe it was the tough guy but not so much of a dick attitude he had. while shane was an utter dick most of the time (at least in my eyes he was)
while it isn't clear, i supposed shane reanimated faster than merle, since shane came back in just a couple of minutes. but merle decayed a lot faster, looking rotten after just what i think was about 2 hours