One of the main issues to be resolved in a Polish Club system is whether to open 1C or 1D on minimum balanced hands with four diamonds. In WJ05 these are opened 1D (though there is some freedom to choose: you might open 1C if the diamond suit was weak). I think that the decision is close but prefer to open 1C on all such hands.
Showing the strength of these hands is not a problem in either method. Both 1C and 1D promise a minimum of about 12 HCP, and this is a fine description of the strength of hands of up to 14 HCP. The real issue is showing shape. If you open 1D then you show 4+ diamonds, whereas if you open 1C you show a balanced hand.
I feel that showing a balanced hand is marginally more useful. As has been said before, the weak NT hand type is the dominant hand type in the Polish 1C opening, and so responder will play for that until proven otherwise. This makes it easy for responder to bid 5-card suits, particularly major suits. Since it is very important to show 5-card suits as soon as possible, and majors are more important than minors, I find that this gives the advantage to the 1C opening. 4-4 fits can often be found later with a take-out double.
Of course, this decision has an effect on the 1D opening as well. It will be opened on a 4-card suit only when the hand is unbalanced. This is a helpful inference in many auctions, in particular making it easier for responder to raise. The resulting opening bid is essentially the same as the Siege 1D opening, and I like to play the Siege 1D agreements as part of Polish Club. This works particularly well with my preference to open 1D on hands in the 18-21 HCP range, since Siege includes sequences to deal with that.
However, the Siege 1D opening also requires you to open 1C on balanced hands with five diamonds (since after a 1D opening bid, opener's 1NT rebid is artificial). Sometimes this is a good thing (when it enables partner to bid a 5-card suit in competition), but sometimes it is not. From the point of view of description and homogeneity, I would probably rather open 1D with the 5-3-3-2 pattern - again, it is usually best to show 5-card suits. In fact, it is sometimes possible to get away with opening 1D on these hands in Siege. With precisely 3=3=5=2 shape, opening 1D works because we can raise a major-suit response to the 2-level. With 3=2=5=3 shape, you might open 1D intending to rebid 2C after a 1H response, but obviously this is best only if the hand "looks" like a minor two-suiter rather than a balanced hand. With 2=3=5=3 shape you are forced to open 1C unless you fancy opening and rebidding in diamonds. So most of the time you will have to open 1C. But fortunately this does still give you the benefits of showing a balanced hand, and freeing up the 1NT rebid after a 1D opening is a great help.
Interestingly, the decision to open 1C on balanced hands with 4 diamonds also often helps when partner makes a negative double. For example, suppose you picked up a minimum 3=3=4=3 hand and were playing a system where this was opened 1D. Now if LHO overcalls 2C and partner doubles, you are a bit stuck for a rebid. Consider the same hand after a Polish 1C opening - if LHO still overcalls a natural 2C and partner doubles, you now have an easy 2D response. You actually benefit from not having shown a suit immediately. If this sounds paradoxical, the explanation is that partner now needs to have support for three unbid suits in order to double, rather than just two. So he can't actually make the double so freely. But then again, the 1C opening makes it easier for partner to bid a suit of his own, and he may be able to do this instead of doubling. Obviously you do lose sometimes - for example if responder has 4=2=4=3 shape after a 2C overcall and would have been able to raise diamonds - but we knew that when we decided to show the balanced hand rather than the diamond suit.