Hi all. I've recently been playing with Unreal 4. I thought I'd share my thoughts on this in comparison with GG.
Unreal first. It's a little unfair in one regard, I've been using GG for years and Unreal for days so I am very, very green with it in comparison.
Pro's
Unreal is way faster. No surprise there. I have yet to actually find a performance limit :0 Everything I have tried has run smooth as butter.
Unreal supports many different game types out of the box. I've literally only played with it for 3 days and have a fairly playable driving game running with a spline based track. I've always wanted to make a driving game and I found this awesome! I've found no other language or tool that has allowed me to do this in all my years of game making and that's a lot!
Testing. Unreal is really fast at this. Click test and often you are in instantly, no reloading the level you see, it's already loaded.
Graphics. Unreal has some seriously wicked visuals. GG isn't even close in this regard. The Mobile demo (for phones) is better than any GG demo or game I have ever seen, and it can run on a phone apparently!
You can add basic geometry and simply drag a material onto it. This is very, very useful. Especially as there is little to no actual meshes by default.
Con's
Unreal's coding is awful. The blueprints system does not compute for me. It looks like Blenders node editor for materials (which is confusing as well, but Unreal trumps it easily). I am too used to old fashioned coding. I can't comment on the C++ side, it may well suit me a lot better.
Unreal has gigantic file sizes. You want to load a few free media packs, prepare for insane download times and also insane setup times!
Building lighting and such takes a long time. You can continue while it's working which is great, but if you do, then the build will likely be invalid anyway and you will have to rebuild it afterwards regardless. This is fairly minor in all fairness to Unreal.
Generally slow at loading up and getting started. If you think GG is bad, try Unreal. You will be amazed at the time it can take to get started.
Unreal is a bit of a nightmare (it seems to me) with adding media. You can add custom stuff obviously, but the included media is very wasteful in this regard. I have to add entire media packages to a project in order to use even one object from it. This may be just me, I am new to it of course. I find this nasty as the media packs take up a lot of HDD space and i am fairly limited in this regard. Having to copy entire media packs for each project takes it's toll on space needed.
If you ever manage to make a game to sell it's going to cost 10% of your profits, or something like that
I haven't really looked into it massively.
Game Guru
Pro's
Easy to use. Obviously.
Less demanding on HDD space. All your media is there, any projects you create can make use of it. You only need duplicates with a standalone.
Easy to add media. I find GG a lot easier to add media in general. Unreal seems overly complex in this area.
GG has default NPC's. Add them and they work, albeit with ropey AI, but they work with no effort on your part. Unreal has no such luxury without buying off the store and as I have never done that, I could be completely wrong. You may STILL need to code your own.
GG has loads of default media included. Unreal does not. You can download free stuff, but as said above it is nowhere near as easy as GG and can take many hours to get AND install.
Scripting is fairly straightforward. Again I have used GG for years, so I am at an advantage. However, Unreal's blueprint system to me, looks like something I will never get into. Ever. Dragging nodes in for variables and such is just not programming for me. I hate it.
If you make a game you have no fee's to pay. It's also far more likely you will actually get to finish a game in GG than Unreal due to it's complexity.
Con's
Speed. GG is slow. I can cripple it in seconds. I haven't managed that in Unreal at all. That being said, I haven't really tried as such. I just see it running smooth as silk no matter what I have tried so far.
Buggy. GG is a little buggy on occasion. Unreal is too, I have crashed it, but overall, it has had less issues than I get using GG for similar amounts of time.
Memory. GG is limited to being a 32bit app and therefore 4 gig max memory limit. I'm pretty sure Unreal has no such issues.
Graphics are nowhere close as said above. A default Unreal terrain looks pretty nice, a GG one looks like a tiled carpet in comparison. Pretty sure unreal has lots of filtering effects as well, which GG doesn't.
So, there you have it. As unbiased as I can be considering I have been using GG a long time. Unreal is surely powerful and looks awesome overall. It has some great features and I can see why it is so well loved by professionals. Game Guru is not as impressive, but is way easier to use. You get a LOT of media by default to get you going and you don't have to search for hours on the store as you do in Unreal.
For the majority of people, Game Guru is the clear winner for the fact you are much more likely to finish a game than in Unreal. Unreal wins in the sense it is way more powerful, but I can't see many GG users jumping ship any time soon. It is just too complicated. Anyone who does, will probably be back to GG quite quickly. I was impressed with it and will dabble every now and then to see if I can get into the coding side more. For now though, I will carry on using GG as my main dev platform, I just find most things easier in it. The race game I made though, that, will keep me looking. I was really impressed with it, curving, windy tracks, fantastic! A shame the coding is like Chinese! The physics can be a little crazy as well, when it goes wrong it sends the car spinning like a crazy thing!
I hope this gives you an idea of the differences between the two engines. I'm impressed with what I have achieved with Unreal in a few days, but getting an actual game made is a different story and would require a LOT of time and work. Game Guru in comparison is a complete doddle and far easier to understand. I've possibly missed pro's and cons for both, but you get the general gist I imagine!
SPECS: Q6600 CPU. Nvidia 660GTX. 8 Gig Memory. Win 7.