Nelo 550 faster than Stellar SEI 1g?

I'm reasonably satisfied with my current boat, a Stellar SEI 1g in heavy construction, but a friend of mine is about to sell his Nelo 550 and I'm wondering if it would be a worthwhile upgrade for me. The boats are the same width (46 cm) but the Nelo is 60 cm shorter and about 4 kg lighter. I'm 75 kg / 178 cm, 40 years old, paddling for about a year and half, comfortable on the SEI in all conditions including rough downwind ocean stuff. Looking for something that will give me a little advantage in flat water racing and that will maybe be a little more nimble riding bumps. Curious to know what y'all think. Thanks. 

If you try the 550 you will probably like it a lot, especially on the flat.
My question would be should you get the convenient boat that your mate is selling or get the best boat for your needs and ability and weight?
You may be better with the ML 550 than the L. You should also consider Swordfish or SEI in a lighter build, or maybe a Carbonology Vault. There are some good posts on this site on intermediate boats for lighter paddlers.

I went from a SEI to a Nelo550L about a year ago, spending about 95% of my time on flat water.
For me, the 550 was immediately faster than the SEI on flatwater (going from 9.7 kph sustained to 10.0 at the time). I am still very happy with the 550 as a flatwater fitness training boat.
The trade-off, again purely from my experience, is that in cross-wind and short, steep windchop on large freshwater lakes, the 550 felt more squirrelly than the SEI.
Didn't try proper downwind or large waves, though...
I terms of size, the SEI cockpit of the Nelo seems slightly smaller than that of the SEI. But keep in mind I am 1.82m and about 90kg.

Thanks for the replies. Good point about the ML vs. L. Wife would murder me if I spent a lot more on a new boat than I get from selling the SEI, though, so I'm limited to cheap secondhand deals. She might murder me even for the thrifty switcheroo I'm contemplating.

I've actually briefly paddled both the 550 L and the ML. I would definitely need to pad the seat in the 550 L, as I do in my SEI, whereas the ML hugs my butt snugly without a pad. I like the flip up bailer on the Nelos better than the always-sticking-out venturi thing on my SEI, which I think adds a little drag.

The other cheap deal I'm considering is a Knysna Rhythm for $1500, which I think would kick ass in flat water but might be too tippy to be fun and practical for me in the ocean. Whichever boat I get I'm going to have to sell the SEI to make room and funds for it, so it has to be a boat that can do everything.

On the subject of bailers, to me there seems no clear winner. The venturi on my Evo is certainly slower than the Epic bailers, but it does not stick out as bad as an open Epic bailer. Overall I found that the Epic bailer is something I need to open/close all the time, while on the Evo, I just leave it either closed in the flat or open in non-flat. Fully flooded boat drains to almost nothing in a few minutes which is not too horrible. Overall feels like less hassle for me. 
I also hear that the foot operated bailers seep some, while a simple plugged hole does not.

I have never paddled an SEI however I did have a second gen SEL for a year and replaced it with a 550 which I still have.  I paddle 90% flat fresh water and in those conditions the speed difference between the two was minimal so I would expect it to be faster than a 1st gen SEI.  

Fitting it a garage is nice as well however what I have noticed is that it does not track as nicely as a longer ski.  In the few times I have paddled in some confused chop, I found the nose has a tendency to move more given its shorter waterline.  However , in short period swell, it fits better in between the waves.  In the end, it really depends on where you are paddling.

I've owned a 550 for about two years now.  Before that I had a Swordfish for several years.  I really like the Nelo.  It's faster on flat water than the Swordfish, and has better glide and body positioning for an effective stroke.  The seat, while wide, feels comfortable and somehow firmly holds your backside in place while allowing rotation.  (I'm 6', 175 lbs.)  I also like that the Nelo feels more lively than the Swordfish - keeps me engaged.  That being said, there seems to be a good amount of secondary stability in the 550--maybe more so than the Swordfish--so it's an excellent stability profile overall.  I don't like paddling super-stable boats that feel like barges on flat water.  Like someone else said, though, a boat choice depends on your typical paddling conditions.  I paddle mostly on the SF Bay, with short fast wind chop about half of the year.  The 550 is great for those conditions, because it's agile and fits nicely on and between short, steep, high-frequency waves.  If I were paddling big ocean swells I'd probably want a longer boat, and if I were paddling only on a flat lake I'd want a faster, skinnier boat.  (I really like the Epic V11 on flat water, BTW.)  But The 550 is not perfect.  The screws on the pedal adjustment cord mechanism rust (I dripped some wax on top to seal them off), and the epoxied bracket for one of the bungee attachments in front of the pedals came off.  Also, the bailer lets a little water in when it's fully closed.  But those relatively minor imperfections aside, I love the 550.