Small green computer sonic transport i5

Fancy casework and branding is the main difference. I think built in storage is larger on some Nucleus, but you can add more to ST if/when you want. If you run along with your library on a NAS, you'll probably prefer to keep your whole library there anyway. 

I've owned a STi5 and run it as Roon Core for about a year and it's great. 2minutes to setup up. It's connected to a switch near the NAS in the room where internet enters the house. Sonore opticalRendu in the listening room takes the ethernet from the wall and output to the DAC. I use Apple devices as Roon Remotes. 
The setup is super-stable and intuitive to use. Although I've never had to re-install my Roon library database, it's easy to set auto-backups in Roon and store them on the ST(or elsewhere, like the NAS). 
Andrew's youtube videos (Small Green Computer channel) are short and simple and will give anyone the confidence needed to implement even if networking, optical, etc. are completely new to you. Cheers,
Spencer

Forgetting about cost is there a big difference between SGC sonictransporter i5 and Roon nucleus? 

I've had mine for 5 months now.  Great purchase.  Much more stable and better sound quality than using my iMac as a roon core.  I also use it with a uRendu

I gots the ST i5. It's a solid no doubt. I think its a great way to go for running The Roons and always recommend it as a solution. I have a Bryston BDP pi as an endpoint and the combo  sounds and works good.I  am always amazed how good Radio Paradise sounds streaming 16/48. But,  If  Manic Moose was not so flaky I might just run the BDP alone.  I might mis The Roons but I do think the MPD in MM sounds a bit more open and full  when doing critical listening.  Bryston is supposedly bringing MM interface up to snuff, I am interested to see which way I goes.

I've had an i5 for four or five years, paired with ultrarendu, running Roon, iPad at the "head."
I like the sounds.  Inevitably, some hiccups with the software, although Roon, which I really enjoy, seems to have gotten considerably more stable over iterations.
Andrew's support is exemplary; he's made it work for me even though I am computer challenged.
I'd say it's a very good way to go, esp. at the pricepoint.

@mofojo  My i7 sT has been solid over the 3 years of usage. Andrew has been terrific all along, now going on six years from my first i5 > i7 AP > current i7.  Let me know if you have specific questions.