space game engine advice

There are very few engines in the world that are good at space visuals, and almost all of them are virtually 100% home-grown.

However, that only matters if you're going for a very high degree of fidelity and/or huge distances. You can easily implement a game that looks and feels "spacey" with Unity or any other engine, although most of the engines out there are going to need heavily modification to get good 3D navigation and other 6DOF features.

It would be useful to have a point of comparison. What kind of game are you trying to implement? What will it look like? How much volume will gameplay typically cover?

I think that Unity could be your best bet. It may take the pro version to get some things, but in general the problem with visual in Unity isn't Unity itself, rather the art assets that people have/don't have. There are some things that UDK can do that Unity can't, but it isn't THAT much of a difference at least in my opinion. In Unity, you still have access to shaders, including your own if you can write them. I think Unity's particle is plenty good for explosions, but you have to use good particle textures or it won't matter what engine you use. Also, scripting code in Unity is actually likely to be faster running than unrealscript(have seen things online confirming that unrealscript is really slow) so you can also code things if you need to do something that is out of the box, and that is how many of the things in the asset store have come about.

I'd say that generally, once you get to a certain point, the engine no longer creates most limitations. For example, the Irrlicht engine has some pretty stupid limitations, like no normal mapped shaders on anything that is animated(due to not having a proper vertex format backend to rendered models), or also a lack of animation blending, or similar things. Unity is for the most part beyond those limits, and at this point it depends on your art as the engine itself is no longer a limiting factor. of course, this does assume you have the pro version.

As far as using the UDK for this...I'd say it is a worthy contender. It has certain things it does better, but it isn't suited to the game type you want. It also has the advantage of being free to use during development, only $99 to use commercial, and only paying the 25% royalties when you get over $50,000 in earnings. This scheme is better for some people, as you get all the features for next to free up until you have enough earnings to not care as much about the 25%. I'd say if UDK enables you to make your game, then it is very worth the royalties. Also, the $99 to start with is much cheaper than the $1500 for Unity Pro. On the other hand, some people prefer to pay the $1500 up front and never risk paying royalties later. The other thing to remember though, is how likely is it that you would reach the $50,000 to have to pay royalties if you used the UDK anyway? This is really another discussion though, but it is something to think about. In the end, this isn't the only thing that would help with a decision, but it is a start.

I'm looking to make a mission based rts. It'll revolve around taking down an established regional power starting with a coup on board a main carrier. Fidelity wise Im aiming for star conflict / eve online levels. In terms of gameplay and scope there will be a fairly standard world map with total war style base development, there will also be battle maps for real time combat. the battle maps will involve a fairly standard rts setup, very homeworld-esque with a stretch goal of allowing the player to pilot 'elite' ships. Units will have special powers (a magic system essentially). Describing the scale is difficult but if you've played star conflict, then thats about the scale. Mind you I'm not looking to finish, only have the visuals and basic logic done, a tech demo.

Unity feels perfect when developing since there doesn't seem to be any preset game in mind within the engine. Aside from the render limitation when using free what really got me poking around was the particle engine. I spent a week or so building a prefab/script that spawns particles and allows me to dynamically adjust settings to make various kinds of weapons fire. Works well, could use some optimization but it allows for huge variety in the kinds of effects weapons effects quickly and easily. Then I duplicated it a few times and watched the framerate plummet. I can work around it by using a single emitter or maybe baking out the different effects (somehow). Other, better, solutions (gl class or line drawing) required full access, so I started looking around again.

Ignoring cost for engines (although that was a good analysis, kburkhart84) I can use unity and be limited in graphics which is my focus, or hack at UDK and take an extended development period. Anyway I'll probably ramble if I continue. Thanks again for any discussion or advice.

Sounds like you don't need a "space" engine. You need an RTS engine with either decent 3-space support (rare, always homegrown) or convincing artwork (much easier to do - and easier to play).