Just like Metal_Rules has done with Roto, I have been pretty much Storm exclusive for the past few seasons, and I have loved the results. As has already been mentioned, sticking with one company makes it very easy to determine where various balls are supposed to fit in relation to one another within an arsenal. Since making that jump, I have had far fewer cases of overlap within my bag.
That being said, there are very good balls to be had from just above every company out there these days. The key is determining what kind of motion you want to see and trying to figure out which company's equipment can give you the motion you seek.
In any event, as for what to add next, I think that depends upon what base you most need to get covered. If you often find yourself without enough ball in your hands because of the age/strength of that old Thunder Road, then I would go with your gut and get the strong piece next. Since you like going straighter, you may want to take something like the Sinister over the Hyper Cell. I say that because I think that the Sinister offers a bit more motion down the lane whereas the Hyper tends to dig in and just roll, roll, roll. If you're a straighter player, you may benefit from the increased entry angle the Sinister can provide.
Now, on the other hand, if your Thunder Road is strong enough most of the time, then you would want to start with the ball that is going to go under it. There are lots of options to consider there. As has already been metioned, the Uproar could work nicely in that role. It's very clean through the front part of the lane, and it offers a pretty strong move at the break point. It should allow you to move in and open things up more. However, if you're not looking to do that and would want something that will allow you to keep your angles tighter, then I would look at something like the Outcry. It's very clean through the heads, but it doesn't seem to want to go sideways off the spot.
When the time eventually comes to look for a replacement benchmark piece, I don't think that you could go wrong with either the Disturbed or the Asylum. Both pieces are smooth and strong.
Obviously, I just focused upon Roto equipment here. Again, I did so because I think it makes things much, much easier to keep straight when you go with pieces from one company. Others have already given you suggestions for possible arsenals from other labels, and that is fine too. You have to figure out what you think is going to be best for you and your game. Good luck picking, and don't be afraid to come back on for help or to let us all know how it went after you do.
Good luck and good bowling!
