Samsung

I just extended my contract on T-Mobile for two years since they gave me a discount on the Value plan but couldn't get a phone upgrade from them. Thinking about just buying the Samsung Galaxy SII somewhere. The T-Mobile guy told me that unless I buy it through them I won't have their 3G or 4G support. Is that really true? I'd rather not buy the phone through them if I don't have to but do want all the features offered.

Would appreciate any input on this.

Thanks