Best current untethered jailbreak/ unlock for 3GS / 4.21 / 5.15.04?

[dbota]

Just got a used iPhone 3GS and erased all pre-existing content by using the iPhone's Settings/ General/ Reset menu and then hooking it up to iTunes and having iTunes recognize it as a new iPhone.

Now I want to jailbreak it and unlock it so it can be used with T-mobile instead of ATT.

From what I am reading in various places on the web, I DO NOT WANT to upgrade the Modem Firmware from its present 5.15.04 to Apple's current 6.15 because it could have negative consequences in the future (but I don't understand the details of that.)

It seems that if I use RedSNow that problematic change of firmware would occur but if I use GreenPoison ( greenpois0n RC6.1 (win) it won't so it would be the preferable tool to use. ??

I am posting because with everything constantly changing in the jailbreak and unlock world from both Apple and jailbreak / unlock tools, it is hard to find what the current truths are.

Would appreciate current advise on the best current untethered jailbreak/ unlock for 3GS / 4.21 / 5.15.04.

I would also like to know if I can or should update the iOS from 4.21 to ... whatever ... either during or after this jailbreal / unlock process and if so how I go about that.

Thnx in advance,

db

NOTE: My first choice would be to do this from an XP PC but I can use an iMac if necessary ...

 

[dbota]

Thanks for the explanation about 6.15.

3 further questions.

Which tools for jailbreaking & unlocking would you recommend?

I googled iFaith v1.4 and all comments were about using it to downgrade from 5.01 to 5.
iFaith v1.4 can also archive my 4.21 for me for later use if needed?

When i do the jailbreak and unlock, can this 3GS be upgraded beyond 4.21 to ...? and is doing so recommended?