I see that when using a tilemap you can edit collision for each "tile", so my question is, is there any advantage to using a separate invisible sprite solid for collision?
(I'm building a 2D platformer)
Not entirely sure what you're asking here in terms of an invisible solid sprite for the collision but a tilemap is used mainly for top-down games to map out large areas with optimised collision. For a 2D platformer you wouldn't use a tilemap, likely just sprites with box collision for platforms.
http://www.gameus.de/Build&Play/ levels are made with tilemaps and finally it depends on how you want to create your platformer (what shapes you want to use for the platforms)
Tilemaps have some advantage (collision groups, easy level creation/load/save)
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Sorry for the confusion:
I'm using the tile-map for the "look" of the level and was going to use a separate sprite (with solid behaviour + invisible) for the actual ground that stops the player falling out the game... but when I discovered that a tile map could have per-tile unique collision I thought why bother with the extra "solid ground"
Oh right, yeah just use the tilemap on its own if you're going to build it like that. No need for an additional sprite for collision.