I'll give you my honest opinion since I have ran both, I'll probably sound bias at the end since I'm sponsored by Alpha but that's ok.

I ran GO engines for 2 years before getting sponsored by Alpha Plus, the first engine ran pretty good, it was a 5 Port, I did had tuning issues but after replacing a few parts the engine ran fine. Then i went through my second engine which was a 7 port and it ran fine, never gave me any issues of any sort except having cooling head fins falling apart but overall it was a good engine.

Now as of Alpha, since the day I got it (Feb 2010) I have never had any issues at all, to date I have about 6 gallons through it and it is very hard to turn it over by hand and it just rips, I have the Z852 and it's the top end version of Alpha and compared to GO's top end engine (7 Port), the Z852 feels with a little more top end (my track has a long straight away) and I do feel I get to the other corner quicker compared to the GO 7 port I had, and it also feels with a lot more bottom end compared to the GO 7 port, sometimes I felt getting pulled away by other engines with the GO 7 port when it came to low end but not with the Alpha, it just has enough bottom and ballistic top.

Non performance details, Alpha wins this one too in my honest opinion, it is just prettier , the box it comes in is very cool, you get a few restrictors (GO too) and covers. The engine itself is just more detailed, like the LSN has markings where it shows you the tuning "hours"

They're both made in Taiwan but excellent engines, but of course I would lean towards Alpha, even if I wasn't sponsored by them, being completely honest, I would go Alpha. That's just my experience with both mills.