Sometimes an internal wastegate will work better than an external type. The issue is kind of complex. For example, if you position an external wastegate on the turbo manifold such that it’s closer to one of the exhaust ports than it is to the others, then it will flow badly. It’s all about flow. If you put the exhaust gas vent from the wastegate right after the turbine (on the downpipe), then it’s bad for the turbo since it creates turbulence and it will increase turbo spool up time after wastegate activation. An internal wastegate can actually be better than an external model if done properly. You would want the inlet to the wastegate to be symmetrical as the turbo inlet so that when the wastegate opens the exhaust gases can find the path through the wastegate easily. Then you want the exhaust gases going out of the wastegate to be placed as far away from the turbine exhaust (main exhaust) as possible. Sometimes a separate exhaust pipe for the wastegate is used.
Internal or external?
On our engines, we don’t need an external wastegate. They are mostly used for engines in excess of 350-400hp. An internal one would do us just fine. However, an external will still work, it’s just not necessary. I personally like the look of an external one. If you are going to get an external one, a small size such as TiAL’s 35mm would be just fine.