I only have experience with Siemens gear so I thought I would draw from the vast amount knowledge and experience from those here that have been at much longer than me so here goes.
If your plant has only Siemens switchgear, that is a clue to the thinking at that site and for that company. I have found that Siemens and GE compete on price and will generally have the best up-front prices, but maybe not all the bells and whistles. Much contractor-selected electrical equipment winds up being GE or Siemens due to the pricing. That doesn't mean it is better quality, just that these companies offer products for the type of "bare-bones-do-the-job-and-no-more" that contractors have to use to get a shot at the bid.
Allen Bradley probably has the most deluxe version for Motor Control Centers. One great asset from Allen Bradley is the CenterONE MCC software program (that you can get from your local AB distributor) that allows you, the planner, to lay out a complete MCC, producing layout sketches, AutoCAD one-line diagrams, parts list, specifications, and the whole package. The CenterONE program will check your layout for any overloads on each MCC vertical section, check your bus bar sizes, and coordinate your breakers and fuses with the main breaker or main incoming bus, make sure you have enough width, depth, and height in your MCC for all equipment, recommend top pullboxes if needed, and when finished, you can Email the package, or print and mail it to your local AB dealer for a very fast quote. The AB MCC can include a lot of equipment that might have to be auxiliary equipment for other brands, such as MCC-mounted AB VFDS, PLCs, Ethernet network in the MCC, built-in power-factor correction capacitors, arc-flash breakers, and so on.
In past projects, the limit for AB is that they do not provide much if any higher-voltage switchgear or transformers above 480 volts, so if you want to get it all in one brand, you would have to go with Siemens or GE. Square D is sort of in-between, having MCCs and also some higher voltage transformers and switchgear. Square D provides more local hand-holding for the engineer or project manager in selecting equipment, than does GE or Siemens. With GE or Siemens, you mostly have to already know what you need.
As Tim said, plan to have generous spares for everything, at least 30% spare breakers and motor starters of different sizes, with the spare motor starters proportional to the ones used in your plant now. In other words, if 80% of the motors in your plant use NEMA size 1 starters, then make 80% of your MCC spare starters also Size 1, and so on for all sizes up to Size 5. Yes you can add them later, but for such a high price that it could be a problem for new projects or for modifying old ones.
The manufacturer's have to compete with each other for initial orders, but NOT for follow-up orders: you are their captive customer and then they have a monopoly condition for your follow-up business. Other brands will not work or fit. I know from past experience that they all take prime advantage of that situation.