I say, make the effort to design original graphics! Two or three good tilesets per era can make all the difference in the quest to make the game look better than amateurish. Whoever said "style" was exactly right: Those graphics don't mesh with the Chrono style at all; they look unwholesomely generic and lifeless. The future tiles are too heavy on white--very grating on the eye--and lack any indication of human culture. It's all much too sterile and devoid of subject matter. They also suffer from not being designed very well...too few colors, and poor use of color. The modern tiles don't have the problem of lacking culture, but they do suffer from the color problem, and they are grating on the eye in their own way.
Sprites and upper tiles have to stand out with some form of distinctiveness. This isn't apparent in the tilesets shown. To achieve distinctiveness you need to use colors more creatively so as to achieve shading and bordering effects that create space on the image (like making fire with red and yellow and orange instead of just orange), and you need to use very bold colors (like the while pixels in Crono's hair) to sharpen the overall representation. Lower tiles, meanwhile, have to stand out not with distinctiveness but with self-continuity and completeness--that is, you ought to be able to see what it is you're looking at just by looking at a single tile. This applies to surfaces like roads, walls, grass, and such.
You prolly already know this, but the reason RPG areas and dungeons have such personality to them, with respect to one another, is that they have their own tilesets. In this way, settings can make use of limited colors and tileset space. Pastoral settings can go heavy on pastoral colors, and can splurge on multi-tiled constructs like large trees and flowers. Dungeons can load up on a million different shades of black and a mixtures of chains, gates, and bricks. The more tilesets you are willing to design, the better your game will look. They should all flow together, however, under a common style. (In other words, if you cross Earthbound graphics with Secret of Mana graphics, I kill you!)
I didn't vote in the poll out of courtesy, but I do think you should consider taking a few weeks to design a homemade tileset and sprite library, and switch Chrono Crisis over to that. Designing a game isn't just about building maps and coding events. You have to do a hell of a lot of prep work, too. Creating your basic graphics library is a part of that.
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