Note that the XLS Extra that I recommended has very little in common with XLS. I think Samson's naming of this line is terrible. They don't have the same cover or core and have very different properties (XLS Extra has approximately 1/3rd of the stretch of XLS).
XLS Extra is a core dependent line with a dacron/poly cover and a dyneema/mfp hybrid core. It's cover can't be stripped and the core exposed like most other core-dependent lines (that is the primary difference between XLS Extra and the more expensive MLX).
XLS is a pretty standard poly cover/poly core line where you use a standard double braid splice. It is competitive with Sta-Set (not X). I rarely use it unless I find a cutoff for a good price, because Samson LS has similar properties for less money.
Samson cheat sheet: (prices are for 5/16" and are from Defender)
LS($0.45) -- cheapest double braid you'd use on a boat. About 3% stretch at 20% of breaking.
XLS($0.58) -- about 10-20% stronger than LS, comes in more color options, same stretch.
XLS Extra($1.05) -- 1/3rd of the stretch of XLS, core dependent splices
MLX ($1.45) -- A little stronger than XLS, stripable cover
Warpspeed ($2.15) -- A lot stronger than MLX, full dyneema core (basically amsteel with a cover)
Ultra-Lite($1.04) -- Closest to MLX, not as strong and it floats and doesn't absorb water. A specialty line for spin sheets (not guys).
Note that strength is expressed in % stretch at % of breaking load. A 5/16" line that is twice as strong with the same stretch will have half of the stretch in practice. An example:
MLX -- 0.9% stretch at 20% of load -- 4500lbs breaking in 5/16"
Warpspeed -- 0.62% stretch at 20% of load, 0.44% stretch at 10% -- 6200lbs in 5/16"
So 20% of 4500lbs is 900lbs. That is about 15% of the breaking strength of Warpspeed. So the stretch for Warpspeed will be around 0.5% at the same load, not the 0.62% stretch of the 20% number.
On smaller boats like my Pearson 28-2 or TakeFive's Catalina 250 being sailed recreationally there is rarely reason to go higher end than XLS Extra. I did use MLX for my spin sheets so that I could strip the covers and because they need to be very low stretch when used as guys.
If you think through the loads you can figure out where to buy fancy line (halyards, guys, reefing lines) and where it's less important because loads are low (main sheet), because the line is grossly oversized for easy handling (jib sheets), or because lengths are short (boom downhaul or cunningham cleated at the mast).
I almost never buy NE Ropes stuff, I've almost always found a better line from Samson at a lower price. The one exception is Regatta Braid, which is a single braid that is easy to splice to dyneema single braid and much nicer in the hand than Samson's budget single braid.