I've been at these exhaust "crossroads" before and I made the less effective choice by going with
flowmaster 40's first. I had the flows for a couple years before I changed my stock h-pipe to an X-pipe with cats. WOW! The X-pipe, even with two cats, increased the sound big time as it opened up the air flow from the engine and allowed the mufflers to resonate more.
This is not a terribly accurate analogy, but bear with me to basically describe the difference in the mid-pipe and the muffler:
1. The stock mid-pipe restricts air flow considerably. An aftermarket mid-pipe (H or X) will increase the air flow volume traveling to the muffler. Two cats are way better than stock, but off-road will give you the loudest sound possibilities.
2. The muffler will change the "pitch" of the exhaust sound. When more air volume is passed through the muffler, the more sound volume is produced. Different mufflers resonate at different rates (rpms), thus some are quieter and some are very loud. When the volume of air is increased, the muffler will produce more sound in the rpm range it was designed for.
3. In audio terms, think of the mid-pipe as the "amp" and the muffler as the "speaker". Put a killer speaker alongside a weak amp and you loose the effect, but match a good amp and speaker and you will get an increase in performance.
4. X and H pipes are different because they send different pulses of air to the mufflers (in basic terms). Therefore, some mufflers sound great with one and not the other, but yet some sound fine with either mid-pipe.
Sorry so long. If it doesn't help.....oh well. I explained this to someone else and they began to understand this process.
Go with a mid-pipe first, then match a muffler to it later on. I'd listen to different H and X pipes as they sound different from each other. From there, it's your personal opinion and your tastes in sound. Mid-pipes are not always expensive.