Arsenal would make a good choice by signing Arthur and Tielemans both. But I believe that they should be focusing on signing especially Tielemans. Because Tielemans' performance at Leicester has been very fascinating until now. He has already drawn the attention of big clubs undoubtedly. Considering this point, Arsenal would make a great addition if they sign him. And as for Arthur, I haven't liked him so far. And I don't know if he can start playing better in case he comes to Arsenal.
If the fund is available, yes they should get both of them. But do not forget that Arsenal is targeting this Brazilian Burno Guimaraes. Arsenal have identified Guimaraes as a potential addition this month with Mikel Arteta keen to improve his midfield options before the end of the window. So now Arthur Melo, Youri Tielemans and Bruno Guimaaraes. And they are all young players. It now looks like the deal for Bruno Guimaraes is proving difficult, so they should be expecting the one for Tielemans to equally take the same shape as I feel it would be difficult to sign a player of his caliber from another rival club like Leicester during a winter transfer window. If these two transfer does not go through then Arthur for me is also superb. Defensively he may not be what Arsenal needs but would be a great addition to Arteta's project. He's just a natural baller.