Which Filter Thread Size Should I Choose?

[doctorbeat]

I'm making some modifications to a digital compact that I picked up cheap on ebay.

I'm going to do an IR conversion, but I'm not going to replace the hot mirror with an internal IR pass filter.

Instead, I'll be using filters over the lens so that I can switch between different wavelengths of IR filter and also use an IR block filter for regular non-IR photography.

The camera has no filter thread, so I'm going to attach a step up ring onto the front of the lens housing to provide one.
I can install either a 30mm or a 30.5mm thread.
Are there any reasons to prefer one over the other, for instance availability of filters etc?

[Slartibartfast]

why not google each type, and see what comes back?

[doctorbeat]

I tried that first, but didn't come up with much.
Wikipedia lists the most common sizes starting with 30.5mm, so I thought maybe that meant that 30mm was an unusual size, but it also said that there are a few different thread pitches in 30.5mm, which is exactly the type of pitfall I'm hoping to avoid.
Anyone got any experience with these small thread sizes?

[nimbus]

Try SRB Griturn, often they can turn something up. They specialise in filters and adaptors.