You can use either (so long as it is clear when your findings were made)
Contrary to other answers here, I see nothing wrong with reporting results in the present tense. When you read old newspapers from a century or two ago, and they write in the present tense, you don't take that to mean that the things in them are still true, and you don't consider it an error on the part of the writer if something they assert is no longer true --- "Even with the arrival of the Benz Patent-Motorwagen, the horse and buggy is still the favoured method of transfort for society ladies".
Any sensible reader of an academic paper understands that the assertions made relate to research conducted at that time and that assertions about the state of the world related to the time and place under consideration in the research. In particular, when you refer to groups of research participants, it is understood that you are referring to a particular group of people as they existed at the time of the research. (If this were not true, how would we ever report age data for such groups?)
I disagree with the other answer here asserting that writing in the present tense makes a claim that something is "eternally true". That is simply ridiculous --- people make assertions in the present tense all the time, and they rarely intend these assertions to contain eternal truths. To the contrary, any sensible reader will interpret writing in the present tense as meaning that the relevant facts are asserted to be correct at the time of the research (i.e., usually about a year or two prior to publication).
In terms of which tense is better, that is contextual, and you will need to use your best judgment as to what sounds clearer and more accurate. Either tense should be legitimate so long as it is clear to the reader (often from context) when the findings were made. Unless there is a good reason to the contrary, present tense will usually be interpreted as referring to the time at which the research was conducted. Past tense is a bit trickier --- depending on context, it might be interpreted in this same way, or it might be interpreted as meaning that the asserted fact was true at some time prior to the research being conducted. If you are using past tense, you should be careful to ensure that you are not implicitly suggesting a contemporaneous change in facts occuring at the time of the research --- e.g., saying "Group A was taller than group B" might suggest that they are no longer taller now.