For one with Digits you have the 1 number across all your devices, with GV you have a different number.
Not if you sign in with the same google account.
Let's see the only main thing I can spot is that Google does sell your number to advertise, I have yet to hear about digits number being sold for that reason.
Another thing Google has been doing this for a while so it's a more stable than digits.
So here is a comparison for you:
- Google Voice is completely free (unless you are making International Call) whereas for DIGITS you need to shed some money monthly.
- For Google Voice, invariable you need to install the Google Voice app or Hangout + Hangout Dialer to make it work whereas in case of DIGITS its integrated in the modern Android phones like Samsung G6, G7, G8 or LG G5. But for iPhone and other phones you still need the DIGITS app.
- DIGITS gives you a true TMObile Telephone number where as GV does not. Ideally, GV emulats one number multiple device scenario since you can ring multiple devices from same GV number by adding those number in your GV account. But to emulate One Device Multiple numbers, its hard to emulate unless you are happy with just two numbers namely your cell number and GV number. Remember a single mobile number can only be added to only one GV account. In DIGITS you can use upto 5 different numbers in the same device.
- In GV you get an extra layer of number if you do not want to expose your cell number. But in DIGITS your cell number will get exposed since the call will directly go via DIGITS and it will be as if you are calling from your TMobile SIM number.
- Many short code SMSes like bank SMSes (say every time you swipe your Chase credit card, you want a text alert from Chase number 24273) may not come to GV. But DIGITS being treated as a true cellular number, this service will work seamlessly.
- GV allows you to record a conversation by *4 which is not present in DIGITS.
- To share a GV number with someone else, you actually need to share your credentials which is a potential risk. But in case of DIGITS you can just add the other person's T-Mo ID and they will get the access. You dont need to share the credentials. This is particular useful in the following scenario: Say you call Internationally a lot and you are a family or in a friends group. In DIGITS you can get a DIGITS Talk and Text and add State Side calling feature to it,. Then you can share the same number with your other family members or to your friends via their T-MO IDs. So SSI feature which costs $15 per month will be shared among few people and your per head per month cost will be less.
- GV allows you Pay-As-You Go cheap International call with call rates starting just at $0.01 per min whereas Pay-As-You-Go rate for T-Mobile International call possibly starts from $1.99 per min or so.
Let me know if that helps you. If I can think of any other points, I shall add to this post.