Profile rails are much more rigid than round rails. Round rails for obvious reasons have zero moment load rating, whereas profile rails with 4 rows of balls have a high rating. The profile rails with 4 rows of balls is what you should look for.

Profile rails are preloaded, so there is no clearance (theoretically negative clearance) which ensures there is no play. This is especially important on the Z-axis where gravity does not effectively preload them.

For the lengths you are suggesting there will be some profile rails on eBay for a good price.

In the end it depends what you want to use the machine for and if you can afford it. Round rails are perfectly adequate for machining wood, composites and aluminium at moderate feedrates especially if they are orientated optimally.

I can't think of any advantages of round rails over profile rails, other than obviously the price!