I have a #5 hand plane and love it, use it nearly every time in go in my shop. Right now im working on a heavy duty shelf, 1.5" thick and 6 feet long. Since im lacking in any stock by those dimensions im doing a considerable amount of jointing. Would a #6 or #7 give me a particular advantage over my #5, I know its longer and will ride over the high spots better, but will that exta length and weight be worth me picking up another plane?
The No. 5 is a very useful plane as you mentioned, but for edge jointing longer does help. No single plane can perform all our planing tasks which is why there are so many design variations.
If you have the money I would get another plane. Which plane may be personal preference.
No. 6 and No. 7 have the same blade, 2 3/8in wide. Useful for surface planing, but not going to help in jointing. The length of these planes will help in jointing. The No. 7 is referred to as a jointer plane, the No. 6 as a foreplane.
No. 6 is about 18in long. No. 7 is about 22in long. Longer sole should help to get the edge straight easier than a No. 5. Some folks like Patrick Leach prefer even longer jointer, a No. 8.
Whether you should get a No. 6 or No. 7 is a bit of personal preference. Longer also means more weight, which can mean getting tired easier.
I have a No. 6 and No. 7 and find it easier to use the No. 6 due to the lighter weight.