I was thinking of buying an Oval concepts road seatpost, but they also have a mountain bike seatpost with the same diameter but a different length for much cheaper price. Is there any difference between the two other than weight?
You've heard the expression "dumb as a post". Seatposts don't know or care where they're used, road, MTB, fixed wheel, it's all the same.
What counts is the diameter and length. There are sometimes other differences in posts made for road vs MTD, such as rear offset, and in some cases MTB posts are built stronger (heavier) in expectation of lots of pounding in use.
If you're willing to accept the weight penalty of the less expensive MTB post, go for it, and spend the savings on something worthwhile.
One caveat: Make sure it won't be too long. Sometimes on a relatively small road frame a very long post will bump into the bottle-cage bolts before you can get the seat low enough. Depends on your frame and your fit. Of course, you can cut off the excess if you're handy with the hacksaw and file, which would take care of the weight penalty, too.
I used to buy mountain bike versions of nicer seatposts because they're longer but are otherwise the same. By doing this I can cut off the marred lower portion of the post and end up with an essentially new road seatpost. Just make sure that you don't cut it too short as a really short post will have almost no resale value down the road. About 10-11" overall length is about right.
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