I don’t know where you got the information that the Sweetwater Guardian ($54) is not field serviceable. It’s very serviceable, provided you have with you the parts you need to work on the thing, which is the same for any water filter. It’s extremely effective, pumps well, and should be considered a near-lifetime purchase with reasonable use. First Need’s Deluxe ($85), meanwhile, uses a clever mechanical process to separate viruses from the water in which they’re traveling, meaning you don’t have to wait for a virus-killing agent to take effect, something the Guardian requires. Kind of depends where you intend to use the thing. In the U.S., in almost all circumstances, a filter such as the Guardian is perfectly adequate, and in fact the Guardian is easier to use and maintain than the Deluxe.
I’d also suggest you look at one other filter: MSR’s Miniworks ($70). This is a first-class filter that I rate very highly in terms of durability and ease of use. It’s also extremely field-maintainable. So throw that in the mix. But you won’t go wrong with any of these devices.