Three types of material genrally used for valve guides
Ampco manufactures the alloy for all the extreme heat racing parts guides, wristpins etc.
SI valves in Burbank has a lot of the Manganese-Bronze made at a local screw machine shop which is 90% of what auto motive machine shops use.
The Phosphour Bronze maintains a bit of lubricity under pressure and is commonly mistaken for the Manganese probably the Manganese has a bit more longevity.
Most of these alloys can be purchased from Mcmaster Carr if you wish. The HEAT on the exhaust side and the squarness of the spring and wiping motion of the Cam on Rocker hasten the wear on the bottom guide in the Porsche. They install at .0022 (clearence) on the exhaust and Porsche considers it to be worn out at .004 you will wear .0015 in 30 to 40,00 miles.
regards

When I did the valves on my 3.6 I researched for days on what valve guides to use. No clear answer anywhere.

The used 3.6 engine I bought had .015 guide wear at 40k miles. It seems as if the phosphor-bronze are a little better at heat dissipation and the manganese bronze are a little better at wear. So maybe for track use vs street use would be the tie breaker.

Although I have only done a few sets of Porsche heads, I have done a ton of other automotive heads and I agree with Steve on the honing. Bronze has to be honed. If you have ever machined bronze it "tears" as you machine it. The hone eliminates this tearing action and the surface of a honed guide is much better than a reamed guide.

Very important......After they are honed, solvent and a good tight fitting bottle brush should be used to remove the honing residue followed by hot soapy water or oil will release the grit only to act as an abrasive paste.

The hone and the bottle brush....
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That bottle brush is very important. It leaves grooves for oil. That oil prevents wear.

I suspect that there is no definite answer for the best material, is that it just dosen't make that much difference. As long as a proper material is used, prepared properly, it will last a very long time. So like I stated before, trust the shop.

By the way you can learn to do this yourself, it ain't rocket science. All you need is proper training by someone who knows what they are doing (very very hard to come by) and a little experience. This is one of the jobs that a home machinist can do in his garage, and do well. So if you spend the time to learn you can do it yourself. Just find some help from a shop that isn't anal and willing to help, or a school, eg the college that Elgin teaches in, near Redwood city, or Saddleback college in Orange CA.

You can buy all the tools for less than a good valve job, to DIY, except for grinding the valves, but your going to buy new ones anyway. PS a not to well known fact (sorry Wayne to give away secrets) is that what a shop pays for valves is much muich less (maybe 1/5) of what you pay.