Whilst it may not be ideal, based upon our current capabilities and squad I think the 343/3421 is more likely.
Our worst case/most likely scenario is that we only have funds for 1 quality starting signing, which is more likely to be Partey or similar in midfield. Any other new signing is more likely to be the result of departures.
Under these circumstances I think the 343 gives us numbers/close support in both defence and attack, as well as then also providing width and roles that we can get the best options out of the squad as it stands, and more so if we do lose both PEA and Laca.
Whilst I do not think the likes of Nketiah, Martinelli, Nelson, ESR are currently ready as a group/combo to start in the roles in a wide 4231, I do think they can be used more readily in a narrower 3 man attack in either a narrow 3 or as in a staggered front 3 as in a 3421.
I also like the natural width given by the 343 and I think this wide position could be suited to any of Saka, Tierney, Kolasinac, AMN, Willock, depending on opposition and balance. Given that one of the criteria is that we sign Partey or similar in midfield, that then improves both the defence and attack in either a twin pivot in a flat midfield 4, or in a wide diamond with both a CAM and DM. I'd also think that this would provide a better framework for Xhaka as the DLP, noting he has also shown a capability to drop into the backline as either FB or CB if needed.
In defence we have seen the likes of Tierney, Kola, Mustafi and Sokratis all play in both a back4 and back3 this season. We know Luiz is better in a back3, and I'd also suggest that both Mari and Saliba can also play as part of a back3. Chambers has played as RB at the start of the season before being injured.