My question is: when and how (whether?) to express the differences in opinions.
Absolutely Yes is the answer. Unless you have some greater out-of-your-control rare situation where even the potential of turbulence or losing your job because of it is so great, you should confront others when you have differing opinions.
The real key here is When and How.
1st the 'When': Every environment is different but some places have weekly meeting or open / round table discussions where bringing up certain topics is the appropriate arena to do this. The main thing you do not want to do is make it like you are belittling or making public some personal design argument that is between you and only 1 or 2 others. The people you are challenging will not appreciate being challenged and maybe even embarrassed in public. For these situations, try to schedule a meeting 1 on 1 with the person(s) in question to detail your thoughts.
2nd the 'How': If you are going to a Senior person, make sure you have all your ducks in a row on backing your thoughts. You can’t just ramble into a senior level persons office saying "All web forms must be stopped, and we must do MVC!". When asked "Why?" and you say, "Well that's what everyone is doing and it is in all the magazines", it will not go far. Be prepared for back-and-forth discussion and being asked about justifying your thoughts on architecture, coding, design, best practices, etc. If you have examples of working code to justify (i.e. a little test harness to prove a thought) this may help as well. The important thing here is to not get into an ego battle or allow the emotions to rise up. This will result in a non-productive meeting and you may not be able to get any of your ideas implemented.
In the end if you have solid, justifiable, and logical suggestions then they should be taken into account. However, also be prepared that there are just some unreasonable people in this world that don't want to listen to anyone but themselves. Hopefully you are not backed into a corner with this type of personality.