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I recently said this in our chat room:
Ita crediderim, sed certus non sum.
A brief discussion ensued about my choice of tense. I wanted to express hesitation, and my gut feeling says that the perfect conjunctive is best for that. I was far from being sure, and I wanted the verb to reflect that. I would like to have more than my intuition behind this: What difference is there in tone between the present and perfect conjunctive when used in such a situation?
Pinkster in the Oxford Latin Syntax (pp. 492ff.) discusses this question, but finds no clear answer. He considers three explanations: (a) there is a difference in meaning, specifically a difference of aspect; (b) the perfect subjunctive in potential clauses is a Graecism due largely to Cicero; (c) there is a pragmatic difference in that perfect subjunctive forms are "milder or less direct". However, Pinkster dismisses all three possibilities saying there is little evidence for any of them.
Woodcock (p. 90) is rather unclear on the issue: he says that "there is no observable distinction in sense between the present and the perfect tenses", but in the very next sentence seems to describe such a difference by continuing "The latter is 'aoristic' and usually refers to the future". The second part of this contradicts Pinkster, who says there is no difference "with respect to the temporal value". The first part ("aoristic") lines up with Pinkster's rejected explanation (a), the aspectual difference (which, for what it's worth, was my own initial guess about this question).
I'm sure there are other sources that discuss this question, but these at least are rather inconclusive. Anyone in search of a thesis topic?
I think you are trying too hard to achieve your objective, by using both a verb in subjunctive mood and a separate (and pleonastic) expression of uncertainty.
The key to a good translation here is to begin at the uncertainty, which you need express only once : in other words, decide on a single way to do it. I would prefer something as simple as Hoc credo, sed incerte.
On the other hand, in answer to your direct question, the difference is merely temporal. If you were to change the verb to, say, opinor, which is less definite than credo, uncertainty would be introduced with the verb still in the indicative : Sic opinor incerte, or Incertus ita opinor, etc., while you might refer to the past with Ita opinabar, sed incerte, or even Hoc suspicabar tantum.
Tuomo Pekkanen's Ars Grammatica (A Latin grammar in Finnish) mentions (§113.3) that
The perfect conjunctive is used as a present time coniunctivus potentialis in the same meaning as the present conjunctive.
The best example he gives is this:
Hoc sine ulla dubitatione confirmaverim. — I might confirm this without any hesitation. (Cicero's Brutus, 25)
This is very close to the phrase used in the question. Therefore I would say that there is no difference between present and perfect conjunctive in this use. There appears to be no temporal difference.
My gut feeling is that the perfect conjunctive conveys a stronger message of uncertainty, but there seems to be nothing to back this up. It seems that it is unknown whether there is any difference; see TKR's answer for a nice summary.
The perfect conjunctive is quite rare in main clauses. According to the cited grammar, it can be used in negative orders (ne timueris!) or as a concession or wish about the past. (Concession: Fueris prudens, pius non fuisti. Wish: Sit ita factum!)
