5' 9", 210#, aggressive skiier...and they told you you should start out on a 172??? I'm 5'6", 165-170# and markedly not an aggressive/largely athletic skier. I demoed all last season, and tried the Prophet 100s several times on several different mountains. Granted, it's a "different" ski than the 98, but I think the skiability is going to be close enough to make a fair comparison between the two lengthwise. I should be riding the 179's, even at my height/weight/ability, but I ended up with the 172s as that was all that was left in stock when I purchased. There is no reason whatsoever I can imagine that anyone in their right mind would steer you away from the 186 - unless you're actually shorter/lighter/less aggressive than your original post indicated.
Now, with all that said, I think you'll definitely like the Prophets. They're a darn good ski by just about all accounts, and they should serve you well in a wide variety of conditions. Hopefully you won't get a lot of chatter on the corduroy or tossed around too much in the crud. They may also be a little bit slower without the extra length, but perhaps not with a proper tune and maintenance. Enjoy!