When we were developing the StudioTips SuperChunk, we discovered that 705 is in fact not a better low frequency absorber than 703... in fact the actual lab tests on the matter show that the opposite is true.
At StudioTips we did a series of tests on several foam and mineral fiber bass traps and found that 3 lbs. pcf density semi-rigid fiberglass boards such as 703 were best in class - 5 lbs. boards such as 705 cost twice as much, weigh twice as much, and do not work any better than 703 - 3lbs. types. Suggestions to the contrary are often made, but never backed up with lab data.
If you use rockwool, try and get something at least about 50% more dense.. around 5 lbs. pcf or more... the 8lbs. flavor mentioned is known to work well.
Also, as mentioned, the denser panels are a bit easier to work with.... particularly when it comes to rockwool.
Use a minimum thickness of 4" - thinner panels will not be broadband [work on low frequencies].
Avoid all types of facings [FRK, foil, paper] unless you seek to reject mid and high frequency absorption. Best practice is adjust your room's contents so that you have a balanced absorption profile across the band using broadband materials. But if you need your panel to exhibit such rejection the easiest and least expensive way to effect this is to simply wrap a plain panel in poly film [6 mil thickness works well and is commonly available].
You can read more about best design practice here:
http://forum.studiotips.com/viewforum.php?f=8
For selecting materials from your local market Bob Golds site is a critical tool. It is based on solid lab data - not anecdotal testing done in Mr. Winer's spare bedroom - you can't carry out material absorption testing outside of a diffuse reverberant field - read more here:
http://readyacoustics.com/index.php?go= ... ustic-data
suggesting otherwise is nonsense.
Use Bob's site:
http://www.bobgolds.com/AbsorptionCoefficients.htm
and the general guidelines given above to identify the class of materials which has the best price in your local market. Do not shop by brand name - but rather by material type and local price.
The data at Bob's site is all certified lab data carried out by manufacturers - you can rely on it. When seeking an indication of effectiveness though, ignore small difference [for example, there really is no effective difference between a measurement of .95 and .91 - read these as equal - these measurements are averages of a very large number of samples as explained in the link above].
If you need more specific advice on best practice for DIY mineral fiber porous absorbers start a thread at StuioTips [home of the SuperChunk]
http://forum.studiotips.com/
or contact Ready Acoustics... we'd be happy to discuss how we make our panels... and why.