Question

Shellsort vs Mergesort, which one to go for?

Answer 1

It depends on a lot of factors...
http://en.wikipedia.org/wiki/Sorting_algorithm#Comparison_of_algorithms

Some questions:
What's the size of the list to sort (will it use a lot of memory?)
Do you want to go for the best case, or will you take into account the average or worst case (is the list mosly sorted or not ?)
Do you need a stable algorithm or not ?
I bet I didn't found all the questions that you may take into account. There is no golden hammer, you have to choose the algorithm best suited to your situation.