I would personally recommend you using Starmie for the same reason that you are choosing between the 2 in the first place:
Starmie Advantages
- Compared to Cloyster, Starmie has better distrubuted stats then Cloyster.
- It is faster then Cloyster
- It can be used a little better because of the fact that one of the most important moves in-game(surf) will not only be useful but it will hit much harder when Starmie uses it
- It has acces to electric type moves and more.
Cloyster advantages
However you must take notice that Cloyster isn't a fantastic Pokemon as the sole reason that it become useful was because of the generation V move, Sshell Smash which it lacks in all other generations. Although it has great defenses, this is easily broken in-game as powerful attacks will tend to break through his defenses and render him useless. Also if you were to bring a cloyster and a Starmie in a fight, Starmie would win because of it's great utility. However you tend to like your Cloyster very much huh?
For this reason I would say choose the one you like best, but if you would want great utility, coverage, power and speed, I would say go for Starmie :)
