You wont know the difference. However I'll try and explain it.

A larger gate will allow more exhaust flow through it. Larger turbos need a larger gate to keep from boost creep.
If you're not making more, or planning to make more then 400whp the 35mm is more then enough.

If you are or planning on making more then 400whp, then go with the 38mm, as it's not much more expensive (if at all) and it'll be worth it not to upgrade later.

Is there any drawback to using the larger gate and making under 400whp? No.