I would ask that you consider in your purchase that a fly rod casts with the upper third or on softer rods, the upper and middle third. But a rod fights larger fish with the butt section.
So there are two considerations here. Casting and fish fighting are separate issues, and both are affected not only by the line rating of the fly rod, but ALSO by the rod action and the rod length.
Softer rods have a softer rod butt for a given rod length. So a softer rod has less power as I defined it on another post
Longer rods have stiffer and more powerful butt sections for the same rod action than a shorter rod. A rod tapers from top to bottom so the butt section of a longer rod will have a thicker and more powerful butt section than a shorter rod of the same action. My 5 wt 10 foot GLX has the power of a 6 or 7 wt 9 ft GLX.
So if you need to cast delicate flies but have the potential for hooking big fish, get a longer rod if the extra length does not hinder you're fishing. That is why the 10 and 11 foot euro nymphing rods are 3 and 4 weights but can handle larger fish than the typical 8 foot 3 or 4 weight.
Casting and fighting fish are NOT just a matter of line weight as has been assumed in this discussion. It should include rod action and rod length.
Finally, I have often heard that a higher line weight fly rod can pull harder on a fish. That is absolutely not true. The degree of maximum pull is determined by the weakest part of the line and leader system. That is the tippet.
The reason is that for maximum pull the rod is taken out of the equation. When we want the maximum pull on a snag, what do we do? Do we pull the rod at an angle to the snag? We do not. We point the rod at the snag and take the fly rod out of the equation, because to pull against the snag with the rod tip risks breaking the rod.
A 3 weight rod with a 10 lb tippet can pull just as hard as an 8 weight rod with a 10 lb tippet when both rods are pointed at the fish. You lose the shock absorption of the rod, but when a large fish threatens to break your rod, lower the rod angle and point the rod at the fish and let the reel drag absorb the shocks.