First, understanding that this is not a site to learn about Truth, but rather to learn about Christianity - what the various teachings are from an academic perspective, the question "Which is true" is off-topic. However, we could take a couple of approaches that would be within the bounds of the site:
- Answering what various denominations believe about the subject
- From an apologetics approach, how do we resolve the apparent conflict?
I'm going to take the latter, and provide the answer the is probably the most commonly accepted.
Before we can address the potential discrepancy, it's important to clarify a couple of key concepts: Mainly what "Biblical inerrancy" means, and what guidelines are used for resolving alleged discrepancies.
First, the basic rules for resolving Biblical discrepancies are documented here: Rules behind resolving alleged Biblical discrepancies
Next, the doctrine of Plenary Inspiration of Scripture allows for authors to use their own literary style, including using allegory, and figures of speech while still allowing for Biblical inerrancy. This means that there can be verses within the Bible that are true on a general common-sense, easy to understand level, yet not technically true. Example: "I'm so hungry I could eat a horse" is a phrase that anyone can easily see is a figure of speech. Anyone can see that it simply means "I am very hungry", but realizes that the "I could eat a horse" is an exaggeration for effect.
The point of the above is that it's sometimes a mistake to be hyper-literal when interpreting individual passages of Scripture. Further, from the perspecrtive of inerrancy, the doctrine is not meant to defend the idea that everything in the Bible is to be taken literally, it teaches that the Bible is a reliable source of truth and doctrine. That said, doctrinally, whether Christ was hung on a tree or a cross isn't doctrinally significant.
One method for resolving this apparent conflict is based on the basic principle outlined above.
It's quite possible that the word "Cross" and "Tree" were used interchangeably, like "Cool", "Neat", and "Awesome" are used in modern slang. In our modern slang, those words have completely different meanings, techically, but in the way we use them, they can mean the exact same thing. Since a cross is made of wood, taken from a tree, it is not unreasonable to assume that either people called crosses "trees". It also be that they called anything that someone was crucified upon a cross, just because the Romans generally did use a cross of one sort or another.
However, all of that said, the historical evidence suggests that Jesus was probably hung on a cross as we'd normally think of it, since it was the normal method of execution at the time, and also based on internal Scriptural evidence. More here: http://www.gods-word-first.org/jesus-christ/jesus-crucifixion-stauros.html
A small snippet of the article, which shows a small bit of possible internal evidence:
The beam that Jesus was made to carry (John 19:17), and that Simon
from Cyrene carried for him after Jesus collapsed in exhaustion (Luke
23:26), was most likely the crosspiece that was later affixed to an
upright pole that was already in place.
