Depends on how you feel about sloops and how you sail. I prefer sloops over the other ships because I like their speed and agility so for me the answer is a resounding yes. Others who prefer either galleons or frigates would probably be better off skipping the sloop and saving for the war frigate or war galleon.
The regular frigate is an excellent little ship and can take on any enemy ship out there and is very tough. If you look
here (scroll down) you'll see that it has some advantages over the war sloop. Better armor and a little more fire power for example. All boils down to how much you car about speed and agility.
If you have owned the regular ships of each class and you prefer one over the others thats the first war class ship you should get. Because the war ships are about the same to drive as their regular class counterparts. If you still can't make up your mind try sailing friends ships to see what they are like.