While both turntables and wyes turn things around, they serve different purposes. A turntable doesn't have to have a roundhouse if its only job is to turn locomotives at the end of a run--this was common in the days of steam, but is less common in the diesel era when you don't have to worry about pushing a tender in front and many locomotives are bi-directional. Roundhouses were used for locomotive storage and maintenance, and often used in places that didn't have turntables--either an enginehouse with several parallel tracks was used, or a more traditional "round" roundhouse was accessed by turnouts in front of the roundhouse area. If you're only using this turntable to turn locomotives, it doesn't need a roundhouse. Be sure, though, to use two lead tracks to the turntable--that way, the turntable can double as a passing track, and the turned locomotive can run back up to the other end of the train to depart in the opposite direction from which it came.
A wye is well-suited for a corner--depending on how sharp your curves are, a wye can fit in a fairly small space. However, the utility of a wye depends on how long its shortest leg is--a compact wye is mainly used just to reverse an engine, but by adding a tunnel or other hidden "off-stage" exit (real or suggested) you can imply that the wye is actually an interchange or the beginning of a branch line set up to take two-way traffic. If you have the room, though, you can use a wye to turn an entire train at once, instead of using the passing track/turntable method mentioned above to turn trains at the end of a line.
