Update July 2014 - Digicel has blocked some VOIP apps including Fring, Viber, WhatsApp. The issue is currently being discussed and I expect it to allowed very soon because of the public pressure to allow it. http://www.digiceltrinidadandtobago.com/en/about/news/public-advisory-digicel-tt-to-block-unlicenced-voip-providers
As a work around, there are other apps like Skype and Google chat which still work or using wifi allows the blocked apps to work.
(Update March 2014 - Digicel is still the winner. The promotions are continuously changing but reception and reliability beat more money to spend and no reception to use it.)
There are free checks you can make via *120# for your total credit/usage for prepaid/postpaid,
Digicel lets you check your voice mail for free.
Prepaid credit expires 6 months after topping up.
Dial *121# to check your remaining postpaid pricing plan minutes and rollover minutes.
You can unsubscribe from Digicel info texts (their various promos and contests, spam) that is sent out to everyone. By default everyone is registered to receive it from both providers but you can choose to stop it by calling in to 100 and making the request on Digicel. You do not have this option on Bmobile.
By default you get a detailed Digicel bill showing every call made for postpaid on Digicel. You can even have paper bills stopped and view bills online only. This saves paper and gas for transport. You have to go in to a Bmobile branch and request a detailed bill that will be emailed to you for free in 24-48 hours on Bmobile and you cannot sign up for e-bills only.
One useful feature from Bmobile is their 'Bligh' feature that gives you $10 credit if your prepaid total goes below $2 and you repay the $10 plus a $1 service charge the next time you top up.Take note that you are only allowed one Bligh at a time and you are charged for each request you make. If you are using a Bligh and you make a second request you will have to pay back $10+$1+$1 now.
Comparison of 4g plans
Do not take my word for it, for all the info go to www.digiceltt.com and www.bmobile.co.tt