G0lddigg@ is spot on! I do use a Mackirk! A "Boss" AU Trap to be specific.
As many others have said the bigger Mackirk models need more water to work properly and the given the Alaskan is one of the biggest models available in the Mackirk sluice range it will need a heck of a lot of water to run it.
If you scale back the models you are looking at to something a bit smaller like a "Boss" or even maybe a "Foreman" you'll probably find that they are a bit more kind in slower water situations (compared to the even bigger ones) but if you can run them in ideal conditions you can still pump huge amounts of material through them.
In the case of my "Boss" my record for buckets of 1/4" classified material run in a day is 36 buckets. It was a bloody huge amount of work and a solid day of digging but it also took a couple hours to run so many buckets through. So I can 100% guarantee you the "Boss" has the capabilities of a reasonably high production sluice.
Ultimately you need to determine what sort of water supply you are going to have available the bulk of the time you are prospecting. If you know that most of the time the water it going to be scarce or running slow, going to a bigger sluice is not a good idea because you simply won't be able to run it. If you know that the creeks or rivers you are working in have good constant supply of water most of the year than a high production sluice might be a great option.
If you've got any questions regarding specifics on the Mackirk's I am more than happy to pass on what I know and I'm sure between all the fantastic forums members you'll get all the information you need.
Stepping up to a highbanker will absolutely cost significantly more that a sluice, there is no arguing that, by the time you build or buy a highbanker, buy a pump, hose and fittings it's going to cost you more than any sluice will. There is pro's and con's both ways you just need to decide where you want to head.
Cheers
Adam