Sorry for the delay slaw. Ideally you want the record pinned to the mat over it's entire surface. Vacuum is the best but complicated. Next is the reflex clamp which I believe was first used by Walker. SME and Kuzma have followed suit. Having a SOTA and an SME I have experience with both systems. The reflex clamp uses a thin washer at the spindle under the record and contacts the record only at the edge of the label. This flexes the record down into the mat and removes minor irregularities. Vacuum will remove more pronounced irregularities. If you store your records correctly you will never have more than minor irregularities unless you buy used records that were stored poorly then I would definitely go for a vacuum table.
What is the effect? Minor but obvious. I think it is because there is some movement of the record in the upper bass and changes in speed and VTA. The music sound more as if it is coming from a tape machine. With a good system the music hovers in air. This effect is improved by these hold down devices. As long as the mat is firm and acoustically inert any material will do. I would never buy a table without vacuum or a reflex clamp. IMHO SME has the best reflex clamp. I have not used a Kuzma.
