I'd recommend the flash.... also get an off-camera shoe cord and a flash bracket, to be able to position it better (than in the camera's hot shoe).
For one thing, for all practical purposes, there is no such thing as a "landscape/portrait" lens. Generally speaking, a landscape lens is a wide angle and doesn't require a large aperture. Conversely, generally speaking a portrait lens is a short telephoto with a larger aperture. So, for landscape a lens such as the EF-S 10-22 might be ideal. For portraiture you might choose a 50/1.4 or 85/1.8, or perhaps a 50/1.8 as a low cost alternative for occasional use.
These aren't hard and fast rules by any means.... People think outside the box and do interesting things with non-traditional tools all the time. However, it's still pretty likely that to shoot landscape and portraiture you are looking at two additional lenses, not just one.
The lens you've got covers moderate wide angle to moderate telephoto range, usable for a wide variety of things. So I'd suggest you stick with that for now and add the flash instead.
The built-in flashes sort of suck, anyway. They are underpowered, drain the camera's battery, and are in the worst possible location for redeye and ugly shadow problems. An accessory flash opens up a whole lot more possibilities and can do a much nicer job.