I know I see some VX graphics in your XP pic. Chibi Sprites are a dead giveaway. The round bush I also recognize from VX while the trees at the bottom of the screen are XP and a recolor of XP. Those trees look quite nice.
For me in XP, I appreciate the extra space as it makes the world feel less like a box and more like a natural enviornment. Filling in that space between is a true pain in the ass. Its where I use Detail Tiles. Like Dirt, Leaves, Sticks, Logs, etc. But without those details, the world feels like its the right size, but somehow 'naked'. I know I can make some great interiors and caves just using the default tilesets, but the outdoor stuff, I struggle with to fill in that extra space. So for me, what I do is to use Custom Tilesets. Mostly just mergers, but they help make the details available so I can fill in the blanks and make it feel natural. I havent ever touched 2k3 but those tiles look detailed enough that I'd be happy with them, which is actually unusual for me.
I can give you an example for XP of what I think helps to fill in those "too much space" areas using Custom Tilesets. Its the Once More, With Feeling Demo in my sig. There are two parts to the demo. Both start off using Merged Outdoor Tilesets. Lightgeist Mountain is was my first attempt complete difficult dungeon, intended more for closer to end of game. The Mountain uses the same exact tileset as the Forest part of the demo, just rearranged differently and using different tiles. The Caves in the Mountain part of the Demo actually just use the Default Tilesets, but I use the Cave Wall tiles excessively to create that feel of detail that I like. It seems to work well for what I want to achieve because it helps to fill in that huge area of "too much space" in caves, but uses very few different tiles. The trick there is just to use the Cave Wall tiles to create cracks all over the place that helps to fill in that empty space.
Caves can usually be slammed out in an hour or two. Outdoor maps for me with all that detail typically take me a couple of days because filling in all that extra space is very time consuming. There is a balance between the two. My intent when creating maps is to knock your socks off, so time for me isnt an issue. But for making a game and getting it out, all that time spent creates a problem because that game will probably never get released if too much time is spent on outdoor maps. You can try out the demo just to examine a way to do details. There is very little story. Its just supposed to be a tour de force for the visuals and mapping technique for XP, so if you take a crack at it, examine the details as opposed to other aspects of the gameplay. Its quite old, but my style there hasnt changed at all, as far as visuals and filling in all those pesky "too much space" spots.
Im not saying you have to or should use "my style" because it has some very serious drawbacks. Chaos Project hosts just 3 Pages of peoples Projects, yet 16 Pages of Scripts for XP alone. Its obvious which type of project people can complete. Scripts arent full games. They could be compared to single maps, instead of multiple maps that any game would need to be finished. Too much work and your project will never be completed. Too little detail and people will criticize your ability to make maps that look half way decent.
There is a lot of negative criticism for RPG Maker games on the internet. When I make my maps, I try to push RMXP as far as I can so people say "Wow! This was made using RMXP! I want to use RMXP too!". I want people that just play RMXP games to not have such a negative opinion of games that are made in RMXP. The goal it to make each player understand that it is up to the person creating the game to create a good game as opposed to expressing a negative opinion of it because of the tools it was made with. For example, the Unreal Engine has been used to create some truly awesome games, and some truly piss poor games. Final Fantasy 13 was made using the Unreal Engine. Its graphically a masterpiece and the gameplay fun, but the story for the game felt like a giant "get from here to there" quest with the necessary ingredients of lettuce and tomato added in because they are required, not because there was a genuine need to put there, but because characters that players can relate with are typically required. The use of Hope as the character that the players were supposed to relate the most with was a piss poor decision. I think players would have enjoyed another Cloud type character. Get here to there, enjoy the scenery, and go back and forth between required story ingredients and killing stuff.
Ultimately, its your project. We'd all love to see you be able to release a project. You will know better than anyone else what you can accomplish with the tools you have. We don't know how much time you have to spend on your project so we cant say "use 2k3 over XP". If you have time, go with XP and a good Custom Tileset and Merger. If you don't want to spend as much time, use 2k3 because it appears that you can achieve a high level of detail in much less time. I understand exactly what you're saying about "too much space" in regards to visual styles, but as was the case with FF13, a game isnt always about the visuals, as 2k3 has its drawbacks too. But after thinking hard about this (obviously) 2k3 may be the way to go so you dont bite off more than you can chew.