Here's some interesting point IMO:

- The "build ground" in DG is more streamlined. Look, Sol has a big grid, and more freedom in choosing specific where to build (if you're really nick-picking). But every single tower is 4 squares flat, so it's not really much effective than a 1-tower-per-square system.

- Tower in DG feels more powerful (probably because their creeps are more ... creepy ). This is pure cosmetic, but I think DG gives a better "feel" when a bullet hits something.

- This is a big point: You can win games and games in Sol with only, say, missile or only laser. Good luck trying to do the same thing with DG. I tried. (this point also has something to do with orbital support, but hey, I got one or two medals with nonsupport and I used only missile).

Other things I like about Defense Grid that I wish were in Sol Survivor:
The ememy has to steal your cores and then escape the map. They don't just grab the goodies and vanish giving you the finger. They have to get in AND get out.

The way the mouse moves the camera around. It just feels better.

The music and animations. I know Sol Survivor is a true 3D game and Defense Grid is a 3D-ish isometric view but the atmosphere just feels deeper and the models for the enemy and towers look more military.

Enemy strength scales sensibly and doesn't suddenly become a brick wall of bosses that can't be beaten. Play any Sol Survivor survival map and you will understand exactly what I mean at around 17 minutes. I guess that's when the Sol Survivor devs simply decided you are supposed to lose and cram it down your throat. They might as well kick you in the taters and spit on you too since that's the way it feels...

Orbital support in Defense Grid is freakin awesome. It's like God himself smashing his fist into the critters and vaporizing loads of them in one fell swoop. It has a very long recharge time though and can only be used once or maybe twice on each map.

Things I like about Sol Survivor that aren't in Defense Grid:
More maps. One of Defense Grid's biggest problems is lack of maps. I'm glad they are doing DLC's now for $0.99 but there just simply needs to be more!

Bigger maps. I love the huge sprawling maps in Sol Survivor. They make Defense Grid's smaller maps seem cramped.

The ability to rotate the camera. Speaks for itself.

Turret selection feels better in Sol Survivor. In Defense Grid it is easy to plop down the wrong turret if you aren't careful.

No annoying lunatic computer in love with raspberries that won't shut up about it. Seriously. How drunk were the Defense Grid devs when they came up with that?

Multiplayer. This isn't really a big deal to me though. Yes it's fun but it's just not why I bought the game. I usually get more into a game's campaign mode for the story and feeling of accomplishment. The survival and multiplayer maps are nice but lack atmosphere or reason.

Turret variation. In Defense Grid you can win most maps using only the gun turret (and creative path shaping) and some temporals to slow the enemy down. I like how Sol Survivor has different resistances on different enemys that make you use the other turrets.