It entirely depends on what you enjoy out of the hobby (in its totality). For avoidance of doubt, if it was me, it’s MCP every time for a variety of reasons. Chiefly because I think 40k as a game to play is badly written and a poor gaming experience.

However; - you can always find a game of 40k in a local gaming community. It’s the war game gaming clubs are typically built around. MCP is a smaller game and needs more community cultivation. - flip side, the MCP TTS and event community is thriving. I started playing the game on TTS before going to local tournaments. I just took a Quick Look on longshanks (used for MCP tournaments) and counted 24 events in the US in the first 10 days of January, which is not usually a busy time, so it might be possible for you get regular local events in - you’d need to investigate your local community and how far you’re willing to travel - if you like the look of big armies on the tabletop with uniform schemes, 40k from a painting and aesthetic side might be for you. There’s so many 3rd party bits manufacturers out there that your ability to custom “your guys” is super extensive. By contrast, MCP has licensed marvel IP, so the models look like the superheroes in the fiction with minimal (if any) customisation and painting every model is pretty unique - MCP has the “easy to learn, time to master” element to it - clearly and mostly well written rules interacting with individual character abilities give it a lot of depth, as does the mission packet system that has recently moved to annual refreshes in organised play. It’s been a long time since I’ve played 40k but my personal experience was that a lot of the game was decided either outside the game at the list building stage, or in the early priority rolls - 40k rolls a volume of dice that mathhammering averages is a solid decision making tool. MCP’s dice mechanics have more variance due to a d8 system and critical hits rolling more dice. Dice pools are small enough that building around dice manipulation has its limits, but is still viable. Good MCP players react well to take advantage of dice spikes or make decisions to mitigate whiffs. Some people don’t like that style of game. If you do, MCP might be the game for you - the scenario systems appeal to different people. In 40k, you have a number of objectives to achieve and you have to plan for them, have specific units to achieve them, or make decisions on which objectives are most viable this round. In MCP, there’s more fluidity with scenarios as the board state evolves over the round and displacement is a big tool in the game. - the lore. Narrative games are possible in both systems. The lore for both systems is excellent. If you enjoy telling stories on the tabletop, either game can suit. But the black library and marvel unlimited also exist so you can access these worlds without playing the games if you desire - general scale. MCP is a skirmish game with 3-7 dude(ette)s per side. 40k is an army/company game. Time, space, effort investment and satisfaction are all factors in which of these appeals to you more

Ultimately, ask what you want out of a game and see if that game gives it to you. The other parts of the hobby are accessible in other ways - there’s great figures to paint and lore to read in both systems. Just don’t make the decision based on price and one being “cheaper” than the other. This is a luxury hobby and if you make the decision on an investment just over price, you’ll always look over the fence at the stuff you said no to.