I have owned everything from a 2 weight up to a 12 weight. For freshwater fishing, I invariably go with the 2 wt, and I catch nearly everything that swims with it. It's harder to cast those size 2 or bigger bass bugs, but it can be done if you modify your casting stroke to match the extra air resistance. For saltwater, I generally use a 9 weight, but I do take the 12 out for shark and tuna. Or the spey for beach fishing.
The point is, if you can handle the modifications to casting, you will be able to handle any fly that you want, within reason, on a 6 weight. If you get a double taper line for it, you'll be able to handle the daintiest dry's in small creeks. It's all about how you approach it. A good quality 6 wt will serve you better than a 4 and a 8 low quality rod. Just get something pretty stiff, with a good backbone to handle the larger fish, and you'll be golden.
By the way, I've landed plenty of 30-40lb stripers on a 4 weight. It was alot of fun, but only for experienced anglers.
The point is, if you can handle the modifications to casting, you will be able to handle any fly that you want, within reason, on a 6 weight. If you get a double taper line for it, you'll be able to handle the daintiest dry's in small creeks. It's all about how you approach it. A good quality 6 wt will serve you better than a 4 and a 8 low quality rod. Just get something pretty stiff, with a good backbone to handle the larger fish, and you'll be golden.
By the way, I've landed plenty of 30-40lb stripers on a 4 weight. It was alot of fun, but only for experienced anglers.