With extract brewing, there's probably little reason to use a chiller unless you want to go to full boils. From what I've read, full boils are definitely an improvement. Assuming you want to go to full boils and that's the driver for the chiller, make sure you are ready to make the full investment.
Going from partial to full boils most likely implies that you have or are going to get:
1) A kettle big enough for a full boil. For 5 gallon batches, you'll probably start with 6-7 gallons, so an 8 gallon kettle would be minimum. I'd go bigger (and I did, I got a 10 gallon kettle). There may be reasons to go even bigger, the primary being that you want to eventually go to BIAB and want to brew high gravity beers.
2) You'll need a way to heat the larger mass. Full boils on your stove would be a challenge, though some gas stoves may be able to do it. I use a turkey fryer burner, but only because it came with a batch of brewing hardware I got used when I started. Buying new I'd get a bayou classic KAB4 (about $85 on Amazon).
3) A way to chill (the subject of this post). I started with a 25' homemade chiller that came with the batch of equipment I got. It was way better than nothing, but still took quite awhile to get the job done. I upgraded to a Hydra and it's incredible. Unless I was going to much larger batches, where an immersion chiller becomes less practical due to the volume of wort, I wouldn't use anything else. It's fast, efficient, easy to clean and is very low risk to harber nasties that would impact the quality of your beer.
If you plan to stick with partial boils, I wouldn't waste my time on a chiller. You can get there reasonably quickly by doing the following:
Chill you top up water to near freezing. I would put my gallons of top up water in the freezer when starting the brewing process. By the time the boil is done, it would have some ice crystals in it but not be frozen. Put the boiled wort in an cold water batch (don't waste ice initially, there's plenty of temperature differential just using cold tap water). After it's chilled some, add it to 1/2 of the expected top up volume that's already been put into the fermenter. Then top up with more of the pre-chilled top up water to reach the desired volume.
Finally, if still too warm, now do the ice water bath. You can add some rock salt to that to make the salt water batch even colder.