Well, there is a lot of pros and cons about which wastegate you get. Some of them come down to cost. The biggest selling point of the Tial 44mm is the v-band clamps IMO. Also the fact that the Tial 38mm is not compatible with race gas (normally, but can be modded). With regard to the Turbosmart 38mm (Ultragate, not the cheaper Streetgate version which I rarely see), it actually flows as much (or more) as the Tial 44mm and can run race gas. It also has a lower profile than the Tial which may help with fitment. Turbosmart is an Aussie product and Tial is American, in case buying from the USA makes a difference to you. If JDM is something you like, the rumor is Helix will be coming out with a WG also (if you fully consider Helix to be JDM). Here is some further buying info (from Turbosmarts website, but applies the same to Tial)...
What should I look for when buying a Wastegate?
a. Temperature rating. Check whether the wastegate has a manufacturer's temperature rating, it will help you in choosing the right wastegate for your application. All Turbosmart wastegates feature a Unique Temperature Isolation zone and are clearly rated by temperature and application.
b. Flow. High valve lift and well-designed flow path are key features to achieving superior boost curves.
c. Diaphragm. It is important that the diaphragm is able to withstand high temperatures and thousands of cycles. Inferior material can deteriorate with time and cause over-boosting. Turbosmart uses High Temperature Silicone Nomex on all its wastegates.
d. Valve Construction. Commonly used two-piece welded valves can break under pressure and high temperatures. All Turbosmart wastegates use high strength, one-piece billet valves.
e. Adapter Systems and Spare Parts. A simple and reusable adapter system is essential for quick and easy installation to a pre-existing or factory turbo system. Turbosmart supports its wastegates with a range of adapters and spare parts to suit most applications.
And the Fact vs Fiction section on Wastegates by Turbosmart...
Fiction: The more powerful the engine, the bigger the wastegate you need.
Fact: This is one of the more popular misconceptions. Use this simple guide before setting out to find the biggest wastegate you can afford.
Big Turbo/Low Boost = Bigger Wastegate
Big Turbo/High Boost = Smaller Wastegate
Small Turbo/High Boost = Smaller Wastegate
Small Turbo/Low Boost = Bigger Wastegate
Fiction: Wastegates don't really need to cope with high temperatures.
Fact: Contrary to the popular belief, external wastegates are usually mounted at the hottest part of the exhaust. It's the place where all the exhaust gases meet, creating extra heat. This combined with the late combustion of unburnt fuel significantly raises the exhaust temperature. Check the temperature rating of the wastegate to make sure it's suitable for your application.
Fiction: The larger the wastegate valve diameter, the bigger the flow.
Fact: While the valve diameter is, without a doubt, an important part of the flow rate, equally important, but often misunderstood, is the importance of the valve travel. When comparing wastegates, don't be guided only by the valve size, check the amount of valve lift. In most cases it's more beneficial to have a wastegate with a smaller valve and long valve travel as this will give you a better control of your turbo resulting in a smoother boost curve.
Of course, with all of that knowledge, it will still probably come down to the fact that Tial is more popular and the Tial 44mm has more pros than the Tial 38mm if the extra price isn't enough of a con.
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