Diffrence in Strings

i am thinking of ordering tenor ukulele strings...but it comes in 2 different choices..high fourth and low fourth..whats the difference and which is more commonly used

"high 4th" is the most common tuning, and is the most traditional.
The uke is tuned GCEA. In high tuning, the "G" is higher pitched than the C.
Your current uke is tuned to high 4th.

"low 4th" is commonly called "low G" tuning.
The GCEA's G is tuned lower than the C. It produces a more lower, guitar-like sound.
The Low G is usually a 'wound' string (silk wrapped around with metal).

It doesn't make a difference in the strumming, as they're still both "G".

See this article for a better explanation. In reality, it comes down to personal preference or as mentioned quite often, get one of each.