I have enough for a 3150 champion but I can't decide if I want to buy one of them or save up for a 4800 one.
I am torn between 
for 3150 and 
for 4800
Any advice on who could potentially be a better pick than the others?
The first, last, and only piece of advice you need on this subject: Buy a champion you have enjoyed playing.
I know you said want to get the "best" champion, but you are going to get the best results playing a champion you like playing, because you will keep playing with them and get better with them. You are not going to do well with a champion if you are only playing them just because D4rkFlameM4str said that Sejuani is OP now or something.
if u farm well karthus if u have good mechanics pick karma if u can land skillshots pick lux malz doesnt really need any skill to play. if u can positon urself to kill there backline talon is good. if u like going even or winning lane hard ahri is pretty good.
