As far as prepaid phones go, the Motorola triumph is basically the top of the line which sucks because their service is atrocious. A few of the bad points with owning the Motorola triumph:
Service:
On my droid2 and droidx i consistently got 3 bars in my house as normal, with this phone i go from one to none sporadically. which i wouldn't mind except that those intervals without service cause delays in receiving emails, text messages, and updates for social networking sites.
Screen:
-This phone has gorilla glass. It come with 2 screen protectors. one to protect it while shipping, and one that if removed drastically improves the response time but MAY void your warranty (rumor but hey better safe than sorry)
-After the first two days, my screen started leaking light. The bottom sides of the screen are back-lit when the screen is off. so the screen is black but theres light pouring through the sides.
-After waking phone up, background looks like a sepia picture for awhile before it loads.
OS:
This phone is running froyo (android 2.2), which is weird because phones coming out at this time are updated to gingerbread. Virgin mobile has relatively NO history with updating its handsets, so if you keep the stock Rom, just know your pretty much using an older version of android.
Keyboard:
The android keyboard doesn't have a built in dictionary. I know, i couldn't believe it either. There is no index for it, trust me. There is an easy fix, you can download swype or another keyboard but the fact that you have to is annoying.
Virgin mobile:
When you turn on a competitors phone it doesnt scream the name of their company so why virgin mobile does it i don't know. you have a great phone but maybe you don't want everyone to know its a prepaid virgin mobile. Thats true of the startup shutdown, default ringtone and notification tone
sound
speakerphone is pretty quiet, hard to hear the person on the other line even when is up to your ear. so if your getting this phone for the unlimited talk, you are going to be disappointed..
If you go with virgin mobile this is the one to get.
buy your phone from target or radio shack.
target has it for $299 but they give you a $50 gift certificate
radioshack has it for $279
the other phones you mentioned are worst, do not go to virgin mobile and get anything other than the triumph, waste of money. virgin mobile and boost mobile are the prepaid units of sprint/Nextel which is weird because you get the best service on sprint, then boost, and lastly virgin mobile. So if your used to the service it might not be as big of a deal for you.
I unfortunately haven't used net10 (they're the ones who still use units for minutes right?)
so i can't really comment on that. but t mobile has a good prepaid plans so if you want a good phone and no contract pretty much the Motorola triumph or t-mobile are your options. If you go t mobiles route i would get the phone refurbished or used to cut back on the price.
hope i helped!