1 or 5 micron. Changing filter every 500gal or so. I buy sediment filters from brs by the case so it's a little cheaper. $100 for 40 5mic or $132 for 40 1mic. I prefer the 1mic because it's what I also use on RO systems.
Wow, you must use a lot more filters than I do. I just change mine out yearly. I've never had one clog up or experienced any pressure/flow loss even after a year with the same filters, but I figure a year's use is plenty on a set, so I change them out at that point regardless of performance.
I typically use the .35 micron sediment filters in my RO setup, but for my dedicated mixing station filter I ended up going with one of the washable/reusable pleated filters.
I'm just hoping to catch any foreign objects that may get in my mixing barrel like dust, lady bugs, dog hair, etc... so I don't need 1 micron filtering for that. A cheap 10 micron washable filter should be more than fine.
My RO setup has a .35 micron sediment filter followed by 2 carbon blocks, a 1 micron carbon block, then a .5 micron carbon block, followed by the RO and then single bed mixed DI. I only change the 3 prefilters yearly. My single canister mixed bed DI lasts well more than a year too. I don't change that yearly though, only when exhausted. Current DI resin was replaced 11 months ago and doesn't show any color change yet. I have pretty good water here. low sediments and my TDS is 100 in summer and 80 in winter, and no chloramines (yet!) so I can't complain about my water too much.