Dave - no expert here, but the heavier (stock weight) balancers tend to idle better. If you have a pretty radical cam and a very light balancer then you'll have a rougher idle.
To my knowledge the lighter balancers don't really confer much of a performance advantage. I could be wrong though.
I just replaced my balancer with a pioneer - was called a high perf. street balancer (a little over a hundred bucks from Summit). It is not SFI. The stock replacement ones are under $100. Pioneer makes a SFI approved for 170 or so.
One thing, you want to get a decent unit - some of the TDC marks can really vary. Also, it is nice to get one that has multiple degree markings on it (the pioneer I have does - the stock replacement ones, whether pioneer or whatever, do not - just the single TDC mark. I have been pretty happy with the pioneer and it did slide on pretty easy.
Hope this helps.
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