If you're worried about vendor lock-in, FPGA/ASIC design is not for you. The design tool market is horrible.
That said, for a novice there are no significant differences in features between Altera and Xilinx.
The place where you might see problems is mostly in simulation, where the big three tool vendors support different language features in SystemVerilog and VHDL-2008. Again, this is not likely to be a problem for a hobbyist/novice.
GHDL has good support for VHDL-2008, I don't know how good Icarus supports the corresponding SystemVerilog.
I wouldn't say it's horrible, it's just not open source (mainly because it's mostly a high-end market). You can easily get by with the free versions of the implementation/simulation tools if you're a hobbyist
That said, for a novice there are no significant differences in features between Altera and Xilinx.
The place where you might see problems is mostly in simulation, where the big three tool vendors support different language features in SystemVerilog and VHDL-2008. Again, this is not likely to be a problem for a hobbyist/novice.
GHDL has good support for VHDL-2008, I don't know how good Icarus supports the corresponding SystemVerilog.