slings can be wonderful or can be a huge pain if the fit is not right. if it is a ring sling, then you can adjust accordingly -- that is what i carried my daughter around in a lot when i was busy chasing my 2yo son around the place (and beyond -- i can still put her in it now at almost 3 years old, but of course she prefers walking everywhere). there are also pouch slings which aren't adjustable but also easier to use because you don't have to fiddle with them to get the sized (just have to make sure you buy the right size). another great thing is mei tais or asian style baby carriers if you haven't tried one of those. there are tons of different kinds and brands and there are also more "commercial" version with buckles instead of tie-on-straps like ergo. these are great because once you figure them out they work on your front or for back carrying. a ring sling or pouch can be used for both as well, but i never felt secure doing anything other than front or side with one personally.
there are also wraps that some people like -- i never did but for younger children i think they do offer a very secure fit as you basically wrap and tie them around you baby/you. some are stretchy (like the moby wrap), and others much less so. but it's another option that some people like, though i never really got the hang of it.
if you live anywhere near a natural baby store or even online store that offers in-person consultation, i highly recommend trying things out in-person first to see what feels right. but do keep in mind it often takes many babies/children a few attempts to get used to carriers, so don't immediately assume it's a bust if there are tears!