IIRC that is the way it works with sense, if you start browsing or sending a file or something, it then automatically switches to H/3G and when idle back to G to save power, however, it actually consumes more battery life this way, more so if you lock the phone and then wake it up a minute later to check out a website etc.
And yes that is normal enough for you to get weaker voice reception if WCDMA is on/active.
Best thing to do, is keep GSM only mode enabled when you are not needing a fast connection (even if you're on wifi) and when you are out and about and need a fast connection, manually switch to WCDMA only mode.
Here is what I recommend you do with your phone to help battery life etc. even more
Leave "best wifi performance" turned off.
Disable any apps that you don't need in settings>apps>click on app>disable at top right
Don't have weather etc. syncing every 30 minutes etc. set to something like 2 or 3 hours.
Manually control the screen brightness as auto makes the battery life worse, install qbright from the market and set a gesture so when you swipe up or down on the homescreen or dock icons this app is opened like the below:
I have it set to currently; night is 0%, indoor 30%, outdoor 50% and sunshine 70%
Keep wifi on as much as possible and set to never sleep
Keep GSM mode selected instead of WCDMA in mobile network settings as one; the voice reception is stronger and better battery life even when connected to wifi I found.
If you aren't going to be using any internet at all for quite while (meaning no need for weather, emails to sync etc.) then turn wifi and mobile network data off.
Only enable WCDMA when you want to browse or stream something (only when wifi is not possible)
Don't use any form of a task killer, if an app isn't responding just use the built in one.