Most of the time the difference is negligible, but there are some exceptions.
Using actors configured for decorative purposes (disabling collisions; no behaviors with update loops; etc.) is roughly the same cost as using tiles.
Actors with always or drawing events attached carry a little more overhead, but you may be surprised what you can get away with.
Using actors to create decorative tiles should be fine. If you're creating collision boxes, though, keep in mind that these tile collision boxes will not be merged into smaller, bigger boxes. This not only creatives additional overhead, but also having separate boxes side-by-side can create a scenario where moving actors get "snagged" by the box corners as they glide over the ground. This effect is very noticeable in platformers. There are ways to get around this, but it's a hassle.