Analysts at Wells Fargo are predicting that T-Mobile would be the one to purchase Sprint, and not the other way around. That is, of course, if a merger does happen between these two companies. That's different from what these analysts stated just a few weeks ago, where they predicted that Sprint and their parent company SoftBank would buy out T-Mobile and combine the two - that was also SoftBank's original plan when they entered the US by buying up Sprint a few years ago.
This also follows on what Deutsche Telekom executives have been saying lately, with its CEO stating that they decide "when, what and how" in regards to any merger or acquisition talk about T-Mobile US. Deutsche Telekom does own a considerable amount of T-Mobile, around 65%. T-Mobile US is also the company's biggest profitable subsidiary, so it's likely that Deutsche Telekom isn't wanting it to go anywhere. T-Mobile and Sprint's executives have also shown signs of interest in merging with each other, with T-Mobile recently praising the 2.5GHz spectrum that Sprint has under their belt. The 2.5GHz spectrum is said to be a big part of Sprint's 5G plans, and that's because it's high-band spectrum, which can offer faster speeds, and that's what 5G is all about.
It would make sense for T-Mobile to be the buyer in this case, since it is indeed the larger company. T-Mobile currently has a little over 72.6 million customers, and Sprint has around 59.2 million customers. Combined, they would have 132.3 million customers, which would put them just behind AT&T and Verizon (within a couple of million). Making it a much more even playing field for the three carriers. Sprint did attempt to buy T-Mobile a few years ago, however the Obama administration's FCC was adamant about keeping four national carriers, to keep competition alive. So Sprint and SoftBank backed off and waited for Trump's team to take office, as they appear to be open to consolidation. And now that the incentive auction is over, the companies can actually talk to each other. During the incentive, they could not have merger or acquisition talks, as it would be unfair to others in the auction.