First let me say thanks for putting a clear question forth, instead of just saying 'which is best' and leaving us asking what the objectives of the map you have made it clear.
Defence here.
My mind is geared towards strategy and lets face it, in most situations the only 'attack based strategy' one needs is selecting all your units and rolling over the enemy with the hopes that your target is one of the things you squish flat. Even in FPS the best attack seems to be nothing more then throwing waves after waves of your respawnable avatars at the objective point. Give also the rate of team play online... well it is near impossable to get a random group of people to attack all at once, instead of individually, let alone to work out things like pincer attacks or sneaking one or two units behind the lines.
Defence though. Normally that tends to have lots of options. In RTS you normally have limitations, such as a small group of units that need to move around to respond to threats and be kept alive. Even in FPS, online or off, you have to place turrets and figure out not just the best places to defend from, but how the attacking force is going to respond to your presence. It becomes a mix of guess work, skill and luck that gives a good challenge indeed.
Now I just wish maps where not so static. It is just too damn easy to plan if you know the enemy can only come through a handful of bottlenecks.